sub isPrime($) {
     $num = shift;
     my ($i,$c);
     for ($i = 2; $i < $num / 2; $i++)
     {
         if ($num % $i == 0){$c += 1;}
     }
     if ($c == 1){return 1;}
     return 0;
}

##</code><code>##

use 5.010;
use strict;

sub isPrime (_)
{
	my $num = shift;
	for my $div (2 .. sqrt $num)     # start at 2, end at sqrt($n)
	{
		return if $num % $div == 0;  # return false if $n evenly divides
	}
	return 1;  # return true if we got through the entire loop
}

for (1 .. 30)
{
	if (isPrime)
	{
		say "$_ is prime";
	}
	else
	{
		say;
	}
}

##</code><code>##

use Carp qw/croak/;
sub isPrime (_)
{
	my $num = shift;
	
	return if $num == 1;  # 1 is not prime
	
	croak "usage: isPrime(NATURAL NUMBER)"
		unless $num =~ /^[1-9][0-9]*$/;
	
	for my $div (2 .. sqrt $num)
	{
		return if $num % $div == 0;
	}
	return 1;
}