For simple problems I usually use something like this:

{
  my @p; BEGIN { @p = (2, 3) } 
  sub nextprime { 
    my $p = $p[$#p];
    DIV: while (1) {
      $p += 2; 
      my $pc = 0;
      while (1) { 
        my $d = $p[$pc++];
        last if $d * $d > $p;
        next DIV unless $p % $d;
      }
      $p[@p] = $p; 
      return $p;
    }
  }

  sub factors { 
    my $n = shift;
    my($pc, @result) = (0);
    while ($n > 1) {
      my $d = $p[$pc++] || nextprime();
      if ($d * $d > $n) { 
        push @result, $n;
        $n = 1;
      } else {
        while ($n % $d == 0) {
          $n /= $d;
          push @result, $d;
        }
      }
    }
    \@result;
  }
}
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The rest of the time I use Math::Big*, sometimes with Pari or (lately) GMP.

Hugo