#!/usr/bin/perl
use warnings;
use strict;
########

sub is_prime {
    my $num = shift;
    my $x = 2;
    my $on = 0;
    while ($num > $x) {
	  if ( $num % $x == 0 ) {
	      $on = 1;
	      last;
	    }
	 $x++;
   }
    return 'False' if $num <= 2;
    if ($num > 2) {
        if (! $on) {
	     return 'True';
        }elsif($on) {
   	   return 'False';
        }
     }
}

sub largest_prime_factor {
    my $i = 2;
    my $num = shift;
    my $large_prime = 0;
    my $prime_factors = [];
    my $factors = [];
    while ($i <= $num - 1) {
	 if ($num % $i == 0 ) {
	     push @$factors, $i;
	 }
     $i++;
  }

   $i = 0;
   while ( $i <= scalar(@$factors) - 1) {
	my $a = is_prime($factors->[$i]);
	if ($a eq 'True') {
  	    push @$prime_factors, $factors->[$i];
	}
     $i++;
  }

   $i = 0;

   while ($i <= scalar(@$prime_factors) - 1) {
	if ($prime_factors->[$i]  > $large_prime) {
	    $large_prime = $prime_factors->[$i];
	}
	$i++;
  }

   return $large_prime;

}

print largest_prime_factor(20), "\n";
print largest_prime_factor(13195), "\n";
print largest_prime_factor(600851475143), "\n";