Regex Modifier: o

gistfile1.pl

#!/usr/bin/env perl
#
use strict;
use warnings;

use Benchmark qw(cmpthese timethese);

my @tests = (
  q{testing 12345 some more stuff},
  q{123 testing some stuff},
  q{this is a really long sentence which is intentionally not giving you a number, so don't even try, it's not going to happen.},
  q{this is another long string.  I promise you at the end there will be a number, but I'm not going to let you have it right now. Ok, Now. 1},
);

my %RE = (
    n => qr/[0-9]+/,
    d => qr/\d+/,
);

cmpthese(500_000, {
    'with o' => sub { /$RE{d}/o for @tests },
    'without o' => sub { /$RE{d}/ for @tests },
});

From: perldoc perlre
o - pretend to optimize your code, but actually introduce bugs

Not quite accurate in the case of using predeclared regex.

              Rate without o    with o
without o 256410/s        --      -41%
with o    434783/s       70%        --