use strict;
use warnings;

my $tri = 1;
my $i   = 1;

while ($i++) {
  $tri += $i;
  my $nof = 2 * scalar(grep { $tri % $_ == 0 } 1 .. sqrt($tri));
  if ($nof >= 500) {
    print "The " . $i . "th triangle number $tri has $nof divisors";
    last;
  }
}

__END__
$ time perl -wl 12.pl
The 12375th triangle number 76576500 has 576 divisors

real    0m26.947s
user    0m25.013s
sys     0m0.027s