our $str = "x " x 1000;

cmpthese -1, {
    single => sub {local $_ = $::str; s/\s+/ /g},
    multiple => sub {local $_ = $::str; s/\s{2,}/ /g;}
};
__END__
           Rate   single multiple
single   1969/s       --     -40%
multiple 3287/s      67%       --