Web Programming with Perl5

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Appendix A
Perl4 to Perl5 Traps

Perl4 to Perl5 Traps

Discontinuance, Deprecation, and BugFix Traps
Parsing Traps
Numerical Traps
General Data Type Traps
Context Traps--Scalar and List Contexts
Precedence Traps
General Regular Expression Traps Using s///
Subroutine, Signal, Sorting Traps
System V OS Traps
Interpolation Traps
DBM Traps
Unclassified Traps

NOTE:

Most of what follows in this appendix is taken directly from the PERLTRAP manual section, which the author maintains. It's included here for your convenience in making the transition from Perl4 to Perl5. The latest revision of this document is always available within the Perl distribution.

Practicing Perl4 programmers should take note of the following Perl4 to Perl5 conversion-
specific traps.

They're generally ordered according to the following list: Discontinuance, Deprecation, Anything that's been fixed as a Perl4 bug,
and BugFix traps removed as a Perl4 feature, or deprecated as a Perl4 feature with the intent to encourage usage of
some other Perl5 feature.

Parsing traps Traps that appear to stem from the new parser.

Numerical traps Traps having to do with numerical or mathematical operators.

General data type traps Traps involving Perl standard data types.

Context traps--scalar, Traps related to context within lists and scalar
list contexts statements/declarations.

Precedence Traps Traps related to the precedence of parsing, evaluation, and execution of code.

General Regular Expression Traps related to the use of pattern matching.
traps using s///, and so on

Subroutine, Signal, Traps related to the use of signals and signal
Sorting traps handlers, general subroutines, and sorting,
along with sorting subroutines.

OS traps OS-specific traps.

DBM traps Traps specific to the use of dbmopen() and specific dbm implementations.

Unclassified traps Everything else.

If you find an example of a conversion trap that is not listed here, please submit it to the Perl5 Porters <perl5-porters@nicoh.com> for inclusion. Also note that at least some of these can be caught with the command-line option, -w.

Discontinuance, Deprecation, and BugFix Traps

Anything that has been discontinued, deprecated, or fixed as a bug from Perl4.

Discontinuance

Symbols starting with "_" are no longer forced into package main, except for $_ itself (@_, and so on).

package test;

$_legacy = 1;

package main;

print "\$_legacy is ",$_legacy,"\n";

# perl4 prints: $_legacy is 1

# perl5 prints: $_legacy is

You can't do a goto into a block that is optimized away. Darn.

goto marker1;

for(1){

marker1:

print "Here I is!\n";

}

# perl4 prints: Here I is!

# perl5 dumps core (SEGV)

It is no longer syntactically legal to use whitespace as the name of a variable or as a delimiter for any kind of quote construct. Double darn.

$a = ("foo bar");

$b = q baz ;

print "a is $a, b is $b\n";

# perl4 prints: a is foo bar, b is baz

# perl5 errors: Bare word found where operator expected

m The archaic while/if BLOCK BLOCK syntax is no longer supported.

if { 1 } {

print "True!";

}

else {

print "False!";

}

# perl4 prints: True!

# perl5 errors: syntax error at test.pl line 1, near "if {"

The meaning of foreach{} has changed slightly when it is iterating over a list which is not an array. This is used to assign the list to a temporary array, but no longer does so (for efficiency). This means that you'll now be iterating over the actual values, not over copies of the values. Modifications to the loop variable can change the original values.

@list = (`ab','abc','bcd','def');

foreach $var (grep(/ab/,@list)){

$var = 1;

}

print (join(`:',@list));

# perl4 prints: ab:abc:bcd:def

# perl5 prints: 1:1:bcd:def

To retain Perl4 semantics, you need to assign your list explicitly to a temporary array and then iterate over that. For example, you might need to change

foreach $var (grep(/ab/,@list)){

foreach $var (@tmp = grep(/ab/,@list)){

Otherwise changing $var will clobber the values of @list. (This most often happens when you use $_ for the loop variable, and call subroutines in the loop that don't properly localize $_.)
split with no arguments now behaves like split(` `,$scalar), which doesn't return an initial null field if $_ starts with whitespace. It used to behave like split(/\s+/,$scalar), which does.

$_ = ' hi mom';

print join(`:', split);

# perl4 prints: :hi:mom

# perl5 prints: hi:mom

Some bugs may have been inadvertently removed. :-)

Deprecation

Double-colon is now a valid package separator in a variable name. Thus, these behave differently in Perl4 versus Perl5, since the packages don't exist.

$a=1;$b=2;$c=3;$var=4;

print "$a::$b::$c ";

print "$var::abc::xyz\n";

# perl4 prints: 1::2::3 4::abc::xyz

# perl5 prints: 3

Given that :: is now the preferred package delimiter, it is debatable whether this should be classed as a bug or not. (The older package delimiter, ` ,is used here)

$x = 10 ;

print "x=${`x}\n" ;

# perl4 prints: x=10

# perl5 prints: Can't find string terminator "'" anywhere before EOF

Also see precedence traps for parsing $:.
Some error messages will be different.

BugFix

The second and third arguments of splice() are now evaluated in scalar context (as the Camel says) rather than list context.

sub sub1{return(0,2) } # return a 2-elem array

sub sub2{ return(1,2,3)} # return a 3-elem array

@a1 = ("a","b","c","d","e");

@a2 = splice(@a1,&sub1,&sub2);

print join(` ',@a2),"\n";

# perl4 prints: a b

# perl5 prints: c d e

The ** operator now binds more tightly than unary minus. It was documented to work this way before but didn't.

print -4**2,"\n";

# perl4 prints: 16

# perl5 prints: -16

Parsing Traps

Perl4 to Perl5 traps having to do with parsing.

Note the space between . and =.

$string . = "more string";

print $string;

# perl4 prints: more string

# perl5 prints: syntax error at - line 1, near ". ="

Better parsing in Perl5.

sub foo {}

&foo

print("hello, world\n");

# perl4 prints: hello, world

# perl5 prints: syntax error

"If it looks like a function, it is a function" rule.

print

($foo == 1) ? "is one\n" : "is zero\n";

# perl4 prints: is zero

# perl5 warns: "Useless use of a constant in void context" if using -w

Numerical Traps

Perl4 to Perl5 traps having to do with numerical operators, operands, or output from same.

Formatted output and significant digits.

print 7.373504 - 0, "\n";

printf "%20.18f\n", 7.373504 - 0;

# Perl4 prints:

7.375039999999996141

7.37503999999999614

# Perl5 prints:

7.373504

7.37503999999999614

Assignment of return values from numeric equality tests does not work in Perl5 when the test evaluates to false (0). Logical tests now return a null, instead of 0.

$p = ($test == 1); print $p,"\n";

# perl4 prints: 0

# perl5 prints:

Also see the "General Regular Expression Traps" section for another example of this new feature.

General Data Type Traps

Perl4 to Perl5 traps involving most data types and their usage within certain expressions and/or context.

Scalar Arrays

Negative array subscripts now count from the end of the array.

@a = (1, 2, 3, 4, 5);

print "The third element of the array is $a[3] also expressed as $a[-2] \n";

# perl4 prints: The third element of the array is 4 also expressed as

# perl5 prints: The third element of the array is 4 also expressed as 4

Setting $#array lower now discards array elements and makes them impossible to recover.

@a = (a,b,c,d,e);

print "Before: ",join(``,@a);

$#a =1;

print ", After: ",join(``,@a);

$#a =3;

print ", Recovered: ",join(``,@a),"\n";

# perl4 prints: Before: abcde, After: ab, Recovered: abcd

# perl5 prints: Before: abcde, After: ab, Recovered: ab

Hashes

Hashes get defined before use.

local($s,@a,%h);

die "scalar \$s defined" if defined($s);

die "array \@a defined" if defined(@a);

die "hash \%h defined" if defined(%h);

# perl4 prints:

# perl5 dies: hash %h defined

Globs

glob assignment from variable to variable will fail if the assigned variable is localized subsequent to the assignment.

@a = ("This is Perl 4");

*b = *a;

local(@a);

print @b,"\n";

# perl4 prints: This is Perl 4

# perl5 prints:

# Another example

*fred = *barney; # fred is aliased to barney

@barney = (1, 2, 4);

# @fred;

print "@fred"; # should print "1, 2, 4"

# perl4 prints: 1 2 4

# perl5 prints: Literal @fred now requires backslash

Scalar String

Changes in unary negation (of strings). This change affects both the return value and what it does to auto (magic) increment.

$x = "aaa";

print ++$x," : ";

print -$x," : ";

print ++$x,"\n";

# perl4 prints: aab : -0 : 1

# perl5 prints: aab : -aab : aac

Constants

Perl4 lets you modify constants:

$foo = "x";

&mod($foo);

for ($x = 0; $x < 3; $x++) {

&mod("a");

}

sub mod {

print "before: $_[0]";

$_[0] = "m";

print " after: $_[0]\n";

}

# perl4:

# before: x after: m

# before: a after: m

# before: m after: m

# before: m after: m

# Perl5:

# before: x after: m

# Modification of a read-only value attempted at foo.pl line 12.

# before: a

Scalars

The behavior is slightly different for

print "$x", defined $x

# perl 4: 1

# perl 5: <no output, $x is not called into existence>

Variable Suicide

Variable suicide behavior is more consistent under Perl5. Perl5 exhibits the same behavior for associative arrays and scalars that Perl4 exhibits only for scalars.

$aGlobal{ "aKey" } = "global value";

print "MAIN:", $aGlobal{"aKey"}, "\n";

$GlobalLevel = 0;

&test( *aGlobal );

sub test {

local( *theArgument ) = @_;

local( %aNewLocal ); # perl 4 != 5.001l,m

$aNewLocal{"aKey"} = "this should never appear";

print "SUB: ", $theArgument{"aKey"}, "\n";

$aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print

$GlobalLevel++;

if( $GlobalLevel<4 ) {

&test( *aNewLocal );

}

}

# Perl4:

# MAIN:global value

# SUB: global value

# SUB: level 0

# SUB: level 1

# SUB: level 2

# Perl5:

# MAIN:global value

# SUB: global value

# SUB: this should never appear

# SUB: this should never appear

# SUB: this should never appear

Context Traps--Scalar and List Contexts

Traps that involve scalar versus list contexts.

List Context

The elements of argument lists for formats are now evaluated in list context. This means you can interpolate list values now.

@fmt = ("foo","bar","baz");

format STDOUT=

@<<<<< @||||| @>>>>>

@fmt;

.

write;

# perl4 errors: "Please use commas to separate fields in file" if using -w

# perl5 prints: foo bar baz

Scalar Context

The caller()function now returns a false value in a scalar context if there is no caller. This lets library files determine if they're being required.

caller() ? (print "You rang?\n") : (print "Got a 0\n");

# perl4 errors: There is no caller

# perl5 prints: Got a 0

The comma operator in a scalar context is now guaranteed to give a scalar context to its arguments.

@y= (`a','b','c');

$x = (1, 2, @y);

print "x = $x\n";

# Perl4 prints: x = c # Thinks list context interpolates list

# Perl5 prints: x = 3 # Knows scalar uses length of list

List--Builtin

sprintf() funkiness (array argument converted to scalar array count). This test could be added to the t/op/sprintf.t tests.

@z = (`%s%s', 'foo', 'bar');

$x = sprintf(@z);

if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}

# perl4 prints: ok 2

# perl5 prints: not ok 2

printf() works fine, though:

printf STDOUT (@z);

print "\n";

# perl4 prints: foobar

# perl5 prints: foobar

Probably a bug.

Precedence Traps

Perl4 to Perl5 traps involving precedence of operations and ordering.

LHS versus RHS when both sides are getting an op.

@arr = ( 'left', 'right' );

$a{shift @arr} = shift @arr;

print join( ' ', keys %a );

# perl4 prints: left

# perl5 prints: right

These are now semantic errors because of precedence.

@list = (1,2,3,4,5);

%map = ("a",1,"b",2,"c",3,"d",4);

$n = shift @list + 2; # first item in list plus 2

print "n is $n, ";

$m = keys %map + 2; # number of items in hash plus 2

print "m is $m\n";

# perl4 prints: n is 3, m is 6

# perl5 errors and fails to compile

The precedence of assignment operators is now the same as the precedence of assignment. Perl4 mistakenly gave them the precedence of the associated operator. So, you now must parenthesize them in expressions like

/foo/ ? ($a += 2) : ($a -= 2);

Otherwise,

/foo/ ? $a += 2 : $a -= 2

would be erroneously parsed as

(/foo/ ? $a += 2 : $a) -= 2;

On the other hand,

$a += /foo/ ? 1 : 2;

now works as a C programmer would expect.

open FOO || die;

is now incorrect. You need parens around the filehandle. Otherwise, Perl5 leaves the statement as its default precedence:

open(FOO || die);

# perl4 opens or dies

# perl5 errors: Precedence problem: open FOO should be open(FOO)

Perl4 gives the special variable, $: precedence, where Perl5 treats $:: as main:: package.

$a = "x"; print "$::a";

# perl 4 prints: -:a

# perl 5 prints: x

Concatenation precedence over the filetest operator?

-e $foo .= "q"

# perl4 prints: no output

# perl5 prints: Can't modify -e in concatenation

Assignment to value takes precedence over assignment to key in Perl5 when using the shift operator on both sides.

@arr = ( 'left', 'right' );

$a{shift @arr} = shift @arr;

print join( ' ', keys %a );

# perl4 prints: left

# perl5 prints: right

General Regular Expression Traps Using s///

All types of regular expression and regular expression operator traps.

s'$lhs'$rhs' now does no interpolation on either side. It used to interpolate $lhs but not $rhs (and still does not match a literal `$' in the string).

$a=1;$b=2;

$string = '1 2 $a $b';

$string =~ s'$a'$b';

print $string,"\n";

# perl4 prints: $b 2 $a $b

# perl5 prints: 1 2 $a $b

m//g now attaches its state to the searched string rather than the regular expression. (Once the scope of a block is left for the sub, the state of the searched string is lost.)

$_ = "ababab";

while(m/ab/g){

&doit("blah");

}

sub doit{local($_) = shift; print "Got $_ "}

# perl4 prints: blah blah blah

# perl5 prints: infinite loop blah...

Substitution now returns the null string if it fails.

$string = "test";

$value = ($string =~ s/foo//);

print $value, "\n";

# perl4 prints: 0

# perl5 prints:

Also see the section "Numerical Traps" for another example of this new feature.
s'lhs'rhs' (using backticks) is now a normal substitution, with no backtick expansion.

$string = "";

$string =~ s'^'hostname';

print $string, "\n";

# perl4 prints: <the local hostname>

# perl5 prints: hostname

Stricter parsing of variables is used in regular expressions.

s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;

# perl4: compiles w/o error

# perl5: with Scalar found where operator expected ..., near "$opt$plus"

An added component of this example, apparently from the same script, is the actual value of the s'd string after the substitution. [$opt] is a character class in Perl4 and an array subscript in Perl5.

$grpc = 'a';

$opt = 'r';

$_ = 'bar';

s/^([^$grpc]*$grpc[$opt]?)/foo/;

print ;

# perl4 prints: foo

# perl5 prints: foobar

Under Perl5, m?x? matches only once, like ?x?. Under Perl4, it matched repeatedly, like
/x/ or m!x!.

$test = "once";

sub match { $test =~ m?once?; }

&match();

if( &match() ) {

# m?x? matches more then once

print "perl4\n";

} else {

# m?x? matches only once

print "perl5\n";

}

# perl4 prints: perl4

# perl5 prints: perl5

Subroutine, Signal, Sorting Traps

The general group of Perl4 to Perl5 traps having to do with signals, sorting, and their related subroutines, as well as general subroutine traps. Includes some OS-specific traps.

Signal Handler Subroutines

Barewords that used to look like strings to Perl will now look like subroutine calls if a subroutine by that name is defined before the compiler sees them.

sub SeeYa { warn"Hasta la vista, baby!" }

$SIG{`TERM'} = SeeYa;

print "SIGTERM is now $SIG{`TERM'}\n";

# perl4 prints: SIGTERM is main'SeeYa

# perl5 prints: SIGTERM is now main::1

Use -w to catch this one.

Sort Subroutine

reverse is no longer allowed as the name of a sort subroutine.

sub reverse{ print "yup "; $a <=> $b }

print sort reverse a,b,c;

# perl4 prints: yup yup yup yup abc

# perl5 prints: abc

warn() Filehandle

Although it always printed to STDERR, warn() would let you specify a filehandle in Perl4. With Perl5 it does not.

warn STDERR "Foo!";

# perl4 prints: Foo!

# perl5 prints: String found where operator expected

System V OS Traps

Under HPUX, and some other System V Operating Systems, one had to reset any signal handler, within the signal handler function, each time a signal was handled when using Perl4. With Perl5, the reset is now done correctly. Any code relying on the handler not being reset will have to be reworked.
Perl5.002 and later uses sigaction() to handle signals under System V.

sub gotit {

print "Got @_... ";

}

$SIG{`INT'} = 'gotit';

$| = 1;

$pid = fork;

if ($pid) {

kill(`INT', $pid);

sleep(1);

kill(`INT', $pid);

} else {

while (1) {sleep(10);}

}

# perl4 (HPUX) prints: Got INT...

# perl5 (HPUX) prints: Got INT... Got INT...

Interpolation Traps

Perl4 to Perl5 traps having to do with how things get interpolated within certain expressions, statements, contexts, or whatever.

@ now always interpolates an array in double-quotish strings.

print "To: someone@somewhere.com\n";

# perl4 prints: To:someone@somewhere.com

# perl5 errors : Literal @somewhere now requires backslash

Double-quoted strings may no longer end with an unescaped $ or @.

$foo = "foo$";

$bar = "bar@";

print "foo is $foo, bar is $bar\n";

# perl4 prints: foo is foo$, bar is bar@

# perl5 errors: Final $ should be \$ or $name

Note: Perl5 does not error on the terminating @ in $bar.
m Perl now sometimes evaluates arbitrary expressions inside braces that occur within double quotes (usually when the opening brace is preceded by $ or @).

@www = "buz";

$foo = "foo";

$bar = "bar";

sub foo { return "bar" };

print "|@{w.w.w}|${main'foo}|";

# perl4 prints: |@{w.w.w}|foo|

# perl5 prints: |buz|bar|

Note that you can use strict; to ward off such trappiness under Perl5.
The quoted string "this is $$x" used to interpolate the pid at that point but now tries to dereference $x. $$ by itself still works fine.

print "this is $$x\n";

# perl4 prints: this is XXXx (XXX is the current pid)

# perl5 prints: this is

Creation of hashes on the fly with eval "EXPR" now requires either both $s to be protected in the specification of the hash name, or both curlies to be protected. If both curlies are protected, the result will be compatible with Perl4 and Perl5. This is a very common practice and should be changed to use the block form of eval{} if possible.

$hashname = "foobar";

$key = "baz";

$value = 1234;

eval "\$$hashname{`$key'} = q|$value|";

(defined($foobar{`baz'})) ? (print "Yup") : (print "Nope");

# perl4 prints: Yup

# perl5 prints: Nope

Changing

eval "\$$hashname{`$key'} = q|$value|";

to

eval "\$\$hashname{`$key'} = q|$value|";

causes the following result:

# perl4 prints: Nope

# perl5 prints: Yup

or, changing to

eval "\$$hashname\{`$key'\} = q|$value|";

causes the following result:

# perl4 prints: Yup

# perl5 prints: Yup

# and is compatible for both versions

Watch out forPerl4 programs which unconsciously rely on the bugs in earlier Perl versions.

perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'

# perl4 prints: This is not perl5

# perl5 prints: This is perl5

You also have to be careful about array references.

print "$foo{"

perl 4 prints: {

perl 5 prints: syntax error

Similarly, watch out for

$foo = "array";

print "\$$foo{bar}\n";

# perl4 prints: $array{bar}

# perl5 prints: $

Perl5 is looking for $array{bar} which doesn't exist, but Perl4 is happy just to expand $foo to "array" by itself. Watch out for this especially in eval()s.
m qq() string passed to eval.

eval qq(

foreach \$y (keys %\$x\) {

\$count++;

}

);

# perl4 runs this ok

# perl5 prints: Can't find string terminator ")"

DBM Traps

General DBM traps.

Existing dbm databases created under Perl4 (or any other dbm/ndbm tool) may cause the same script, run under Perl5, to fail. The build of Perl5 must have been linked with the same dbm/ndbm as the default for dbmopen() to function properly without tieing to an extension dbm implementation.

dbmopen (%dbm, "file", undef);

print "ok\n";

# perl4 prints: ok

# perl5 prints: ok (IFF linked with -ldbm or -lndbm)

Existing dbm databases created under Perl4 (or any other dbm/ndbm tool) may cause the same script, run under Perl5, to fail. The error generated when exceeding the limit on the key/value size will cause Perl5 to exit immediately.

dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";

$DB{`trap'} = "x" x 1024; # value too large for most dbm/ndbm

print "YUP\n";

# perl4 prints:

dbm store returned -1, errno 28, key "trap" at - line 3.

YUP

# perl5 prints:

dbm store returned -1, errno 28, key "trap" at - line 3.

Unclassified Traps

Everything else.

require/do trap using returned value.
If the file doit.pl has

sub foo {

$rc = do "./do.pl";

return 8;

}

print &foo, "\n";

and the do.pl file has the following single line

return 3;

running doit.pl gives the following

# perl 4 prints: 3 (aborts the subroutine early)

# perl 5 prints: 8

The same behavior occurs if you replace do with require.
As always, if any of these are ever officially declared as bugs, they'll be fixed and removed.

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