#!/usr/bin/perl
use strict;
use warnings;
##
my $max = 0;

sub prime_factors {
    my $x = shift;
    my @factors;
    for ( my $y = 2; $y <= $x; $y++ ) {
        next if $x % $y;
        $x /= $y;
        push @factors, $y;
        redo;
    }
        return @factors;
  }

foreach my $num (20,13195,600851475143) {
    my @factors = prime_factors( $num );
    foreach my $p (@factors) {
	  if ($p > $max) {
	      $max = $p;
	  }
    }

    print "Number: $num   Largest Prime Factor: $max\n";
}
-bash-4.1$ time ./large_prime.pl
Number: 20   Largest Prime Factor: 5
Number: 13195   Largest Prime Factor: 29
Number: 600851475143   Largest Prime Factor: 6857

real	0m0.004s
user	0m0.002s
sys	0m0.002s