use Benchmark;

my @words = map { chomp; $_ } (<DATA>);

my $alpha = '[a-zA-Z]';
my $alnum = '[a-zA-Z0-9]';

timethese(2000, {
   'Without /o' => \&testsub,
   'With /o'    => \&testsubo,
});



sub testsub {
  my $count = 0;

  foreach (@words) {
    $count++ if(/^$alpha$alnum+$/);
  }
  return $count;
}


sub testsubo {
  my $count = 0;

  foreach (@words) {
    $count++ if(/^$alpha$alnum+$/o);
  }
  return $count;
}

__DATA__
1500
words
one 
per 
line