There are many half-baked methods used for properly testing whether or not a variable is defined. Following are a few examples of some good and not-so good methods with explanations.

if (exists $opts{'primary'} || exists $opts{'secondary'}) {
    $some_value = $opts{'primary'} || $opts{'secondary'};
}

The above logic is clean and easy to follow at first glance, although it may not be the most efficient way of executing the appropriate decision making block.

Considering the above is equivalent to the following block, you may notice a somewhat redundant conditional checking that ensues:

if (exists $opts{'primary'} || exists $opts{'secondary'}) {
    if ( exists $opts{'primary'} ) {
        $some_value = $opts{'primary'};
    } else {
        $some_value = $opts{'secondary'};
    }
}

Another added benefit is that if your value is “0″ or empty, it may actually not get assigned if you use the simple one line conditional assignment:

$some_value = $opt{’primary’} || $opt{’secondary’};

The “primary option” would be skipped if it is equal to “0″…which is in-fact a completely valid value, and demonstrates the “falsey-ness” and “truthy-ness” of specific values…more on that later.

Thanks to “Ani-_” in #perl for setting me straight on conditional checking.

So, to some it all up in one bullet-proof conditional block for assignment we write the following, which will also turn out to run faster because there is only one conditional check:

if ( exists $opt{'primary'} ) {
    $some_value = $opt{'primary'};
elsif ( exists $opt{'secondary'} ) {
    $some_value = $opt{'secondary'};
} else {
    return false;
}

As explained on perl monks and Pete Kruckenberg’s post…I needed a similar function that returns an intersection between two arrays in Perl.

#!/usr/bin/perl -w

use strict;
use Data::Dumper;

my @array1 = (1, 2, 3);
my @array2 = (2, 3, 4);
my %original = ();
my @isect = ();

map { $original{$_} => 1 } @array1;
@isect = grep { $original{$_} } @array2;

print Dumper(@isect);

#outputs
#$VAR1 = 2;
#$VAR2 = 3;

The above code works with duplicates, meaning that duplicates in @array2 will be added as individual elements in the array @isect.

Perl::Critic is an excellent way to not only validate your Perl source code, but to learn some tips from the Perl monks who have contributed to the Perl::Critic documentation.

I uploaded one of my latest CGI scripts written in Perl to see how good my Perl chops are…turns out I picked up a few tips that I was unsure about in my previous Perl scripts.

A few simple techniques can be used to make your code more standardized in the event that your are not the only person reading it.

The Perl Critic web site has a nice usable web interface for uploading source code and checking its validity — although for obvious reasons if you’re working on enterprise code you should probably download an install the Perl::Critic CPAN module on your own server.