ofcourse there are several ways of checking or testing primality of integers, I used Trial division method. There are other better method I believe. Check below:

use warnings;
use strict;

is_prime( $ARGV[0] );

sub is_prime {
    my ($val) = @_;
    my $num_sqrt = int( sqrt($val) );
    my @factor;
    for ( 2 .. $num_sqrt ) {
        if ( ( $val % $_ ) == 0 ) {
            push @factor, $_;
        }
    }
    my $count = () = @factor;
    print $val, " is a Prime Number" if $count == 0;
}
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And 2 was used here because 1 is not a prime number by defination, which state that a prime number must have two factors: 1 and itself, but 1 has only itself! too bad for 1 though.
Also look into good advice given by tobyink under other suggestion on this post