use Benchmark 'cmpthese';
use strict;
use Regexp::Keep;

my @strings = ('one two three four', 'two three four five', 'one three five');
my @copy;

sub replace { s/(o+ )three/$1/       for @copy=@strings }
sub keep    { s/o+ \Kthree//         for @copy=@strings }
sub two     { /o+ (?=three)/g and s/\Gthree// for @copy=@strings }

cmpthese(-3, {
  'replace' => \&replace,
  'keep'    => \&keep,
  'two'     => \&two
});

replace;
print "Replace: ", join("\n", @copy), "\n";
keep;
print "Keep: ", join("\n", @copy), "\n";
two;
print "Two: ", join("\n", @copy), "\n";