When working with Apache::Registry it is very easy to create accidental closures.
This is due to the way Apache::Registry makes a fake package for your script, as I'll demonstrate in this tutorial.

The only indication that something is amiss (other than the unpredictable behaviour) will be the following line in your error log: Variable "$foo" will not stay shared at ...

The following code demonstrates the problem with accidental closures when using Apache::Registry.

use strict;
use warnings;

my $foo = 5;

print "Content-type: text/plain\n";
print "Content-disposition: inline; filename=foo.txt\n\n";

printf "Package: %s\n", __PACKAGE__;
printf "[%s] Before: %s\n", $$, $foo;
badness(5);
printf "[%s] After: %s\n", $$, $foo;

sub badness {
    my $val = shift;
    printf "[%s] badness: %s\n", $$, $foo;
    $foo += $val;
}
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Apache::Registry will take the above code and create a new package for it based on the ServerName and the name of the script, and then wrap the code in a sub handler {} block.

If your script is running on "foo.com" and is named "test.pl", then this is what the above code will look like after Apache::Registry is done:

package Apache::ROOTfoo_2ecom::test_2epl;
use Apache qw(exit);
sub handler {
#line 1 /www/foo.com/test.pl
use strict;
use warnings;

my $foo = 5;

print "Content-type: text/plain\n";
print "Content-disposition: inline; filename=foo.txt\n\n";

printf "Package: %s\n", __PACKAGE__;
printf "[%s] Before: %s\n", $$, $foo;
badness(5);
printf "[%s] After: %s\n", $$, $foo;

sub badness {
    my $val = shift;
    printf "[%s] badness: %s\n", $$, $foo;
    $foo += $val;
}

}
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First run:

Package: Apache::ROOTfoo_2ecom::test_2epl
[13520] Before: 5
[13520] badness: 5
[13520] After: 10
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Second:

Package: Apache::ROOTfoo_2ecom::test_2epl
[19331] Before: 5
[19331] badness: 5
[19331] After: 10
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Third:

Package: Apache::ROOTfoo_2ecom::test_2epl
[19331] Before: 5
[19331] badness: 10
[19331] After: 5
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Fourth:

Package: Apache::ROOTfoo_2ecom::test_2epl
[19331] Before: 5
[19331] badness: 15
[19331] After: 5
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Notice how the number within the badness sub is increasing for each process, but the $foo that is seen by the instance script is never modified after 'badness' after the first execution for that process.

This is because the badness function is actually an inner function now, and it keeps a reference to the instance of $foo that was created for the first run.

Edit - example of how to avoid this issue added, per rhesa's suggestion

Thankfully it is easy to avoid these problems once you know why they occur.
Tips:

• Keep your toplevel script minimal
• Subroutines should only use the variables that were passed
• Encapsulate behaviour in supporting objects

Example of a working alternative:

use strict;
use warnings;

my $foo = 5;

print "Content-type: text/plain\n";
print "Content-disposition: inline; filename=foo.txt\n\n";

printf "Package: %s\n", __PACKAGE__;
printf "[%s] Before: %s\n", $$, $foo;

badness(\$foo, 5);
badness(\$foo, 5);
printf "[%s] After: %s\n", $$, $foo;

sub badness {
    my ($foo,$val) = @_;
    printf "[%s] badness: %s\n", $$, $$foo;
    $$foo += $val;
}
[download]