perlfaq6:

In versions 5.6 and later, Perl won't recompile the regular expression if the variable hasn't changed, so you probably don't need the /o option. It doesn't hurt, but it doesn't help either.

perlop:

... the largely obsolete /o ...

The bottom line is that using /o is almost never a good idea.

Then what about something like benchmark below (this is 5.42, and I _do_ see a small but noticeable performance bump with real code which does some heavy lifting when parsing. I don't think any variables have changed; just wanted to split pieces of a regex into separate variables for clarity):

use strict;
use warnings;
use feature 'say';
use Benchmark 'cmpthese';

my $pat = qr/\d+/;
my $s = '123abc' x 1e6;

cmpthese -1, {
    foo => sub { 1 while $s =~ /\G $pat \D+ /cgx },
    bar => sub { 1 while $s =~ /\G $pat \D+ /cgxo },
};

__END__

          Rate  foo  bar
foo  1213522/s   -- -88%
bar 10345112/s 752%   --
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