To expand on what thraxil has said...

This probability and permutations exercise could be written as:

#!/usr/bin/perl -w
use strict;

my $DAYS_IN_YEAR = 365;

print "\n# of People \t Birthday Match Likelihood\n" . '-'x80 . "\n";

for my $people ( 2 .. 40 ){
    my $tmp_days = $DAYS_IN_YEAR;
    my $probability=1;
    for ( 1 .. $people ){
        $probability *= $tmp_days--;
    }
    $probability /= $DAYS_IN_YEAR ** $people;
    $probability = ( 1 - $probability ) * 100;
    print "$people \t\t $probability %\n";
}

exit;
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