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use Benchmark 'cmpthese';
use strict;
...
print "Keep: ", join("\n", @copy), "\n";
two;
print "Two: ", join("\n", @copy), "\n";

          Rate replace    keep     two
replace 4739/s      --    -15%    -26%
...
Two: one two  four
two  four five
one three five

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

sub two     { /o+ (?=(three))/g and substr($_, $-[1], $+[1]-$-[1], '')
+ for @copy=@strings