our $str = "x " x 1000;

cmpthese -1, {
    single    => sub { local $_ = $::str; s/\s+/ /g; },
    multiple  => sub { local $_ = $::str; s/\s{2,}/ /g; },
    translate => sub { local $_ = $::str; tr/ //s; },
    capture1  => sub { local $_ = $::str; s/(\s)+/$1/g; },
    capture2  => sub { local $_ = $::str; s/(\s){2,}/$1/g; },
};
__END__
              Rate  capture1    single  capture2  multiple translate
capture1    1321/s        --      -71%      -82%      -84%      -99%
single      4567/s      246%        --      -37%      -43%      -97%
capture2    7244/s      448%       59%        --      -10%      -95%
multiple    8071/s      511%       77%       11%        --      -95%
translate 160627/s    12057%     3417%     2117%     1890%        --