tachyon gave a good soltution. Good for if you're getting the data from someplace else. If it's just for your own sake of readibility you can use underscores (_) instead of commas.

#!/usr/bin/perl

use warnings;
use strict;

my $num = 1_002_345;

my $total = $num + 1000;
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And tachyon commify will work, but the one provided in the Perl Cookbook 2.17 is more efficent.

sub commify {
    my $text = reverse $_[0];
    $text =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g;
    return scalar reverse $text;
}
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