You really should use a smarter algorithm. You are testing a huge amount of numbers that can't possibly matter. JavaFan above has the right idea.

Here's a prime testing function I was noodling around with for comparison. It tests for primality up to 2**32 in about 1/10 sec on my not very fast computer. (Still unrealistically slow for large numbers of tests but ok for the odd single test here and there.)

Updated: fixed error

use warnings;
use strict;
use Time::HiRes qw/ tv_interval gettimeofday/;

my $time0 = [gettimeofday];

# simple routine to test for primality.
# don't expect sensible results for numbers above 2**32
# on 32 bit OSs.

my $test  = shift @ARGV || 2**32 - 5;    # number to test
my $max   = $test**.5;                   # square root of number to te
+st.
my @primes;
my $isprime;

@primes = ( 2, 3 );                      #seed the primes array

# generate the prime test array.
# save it and load it from a file for faster speed.

my $this = $primes[-1];
while ( $this < $max ) {
    $this += 2;
    my $sqrt = $this**.5;
    for (@primes) {
        if ( $_ > $sqrt ) {
            push @primes, $this if $this < $max;
            last;
        }
        next if $this % $_;
        last;
    }
}

for (@primes) {    # test number
    next if $test % $_;
    $isprime = 1;
}

my $elapsed = tv_interval( $time0, [gettimeofday] );

print "$test is ", ( $isprime ? 'not prime.' : 'prime.' ),
  " In $elapsed seconds.";

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In reply to Re: is_a_prime test? by thundergnat
in thread is_a_prime test? by Anonymous Monk

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