At work, I was given a code to add functionality. You can probably imagine - a CGI script that shows a list, some of whose elements might be selected. The part responsible for handling the selected elements confused me (simplified):

for my $id (@selected) {
    $list{$id}{selected} = 1;
}

for my $key (keys %list) {
    print qq(<li name="$key");
    print ' class="selected"' if $list{$key}{selected};
    print ">$list{$key}\n";
}
[download]

Wait, what's going on? We test for $list{$key}{selected}, so $list{$key} is a hash reference. Then, we print $list{$key} - but in the output, we do not get any HASH(0x2561a68), but the correct goods' names. I asked a coworker, and she told me this part of the script was "dark magic"; she had debugged it once and still remembered the value was coming out even if it should not. I played a bit with the script to remember I have already seen a similar behaviour: of course, the script does not use strict! Try yourself:

#!/usr/bin/perl
# Script part 1.
use warnings;
use strict;


my $code = << '__CODE__';
    %list = ( scalar => 'Plain',
              ref    => 'Overwritten' );
    $list{ref}{selected} = 1;

    for my $key (keys %list) {
        print "$key: $list{$key}";
        print " - Selected" if $list{$key}{selected};
        print "\n";
    }
__CODE__


{   no strict;
    my %list;
    eval $code;
    print "Wow: $Overwritten{selected}\n";
}
[download]

The last print explains what happens: Perl sees you are trying to use $list{ref} as a hash reference, while its value is the string "Overwritten". It therefore creates a hash of that name for you (even more funny if the string contains spaces).

So far, so good. Lesson learnt: the people who tell you "use strict" know what they say. I tried to explain to the coworkers what the problem was, but those not familiar with Perl were not able to understand.

But then, I thought to myself: Is it possible to make the code work even under strict? tie and overload would probably be useful, as I remembered from Programming Perl. And after several minutes, I was able to run the code under strict. You can test your skills before revealing my solution:

Just for completness, this is the "common" behaviour I expected at the beginning:

# Script part 3.
{   my %list;
    eval "$code;1" or die $@;
}
[download]

Note: The three last code examples should be kept in one file.