I don't want to run down Regexp::Keep, because I think it's nifty. But I do want to point out a few things:

1. It doesn't provide any help with variable-width negative lookbehind (which the OP thought was needed)
2. Depending on the regexp, it can be faster or slower than the two-regexp method.
3. It's slightly broken (see below)

Here's some benchmarking code:

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";
[download]

As written, I got these results:

          Rate replace    keep     two
replace 4739/s      --    -15%    -26%
keep    5587/s     18%      --    -13%
two     6386/s     35%     14%      --
Replace: one two  four
two  four five
one three five
Keep: one two four
two four five
one three five
Two: one two  four
two  four five
one three five
[download]

Note that keep yields different output: it doesn't keep the space. If I move the \K to follow the "t" in "three" (or anywhere in "three" -- even at the end of the pattern!), it doesn't affect the output.

If I change the o+ to an e+, keep wins by a hair. If I change it to an r (so that nothing matches or is substituted), replace wins and two loses.

The lookahead is necessary for non-global replacement. For global, the subs can look like this:

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 }
[download]

and, in addition to being much neater, keep wins by a hair.

Update:Interestingly, this one-match alternative is significantly slower -- about a third slower than replace.

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