use Benchmark 'cmpthese';
my $str = "alphabet X alphabet" x 100 . "junk at the end" x 10;

cmpthese(-5, {
  # is this the last X?
  last => sub { my $x = $str; $x =~ s/X[^X]*$/X/ },

  # capture up to the last X and replace it with itself
  capt_repl => sub { my $x = $str; $x =~ s/(.*X).*/$1/ },

  # match up to the last X, then remove everything after it
  rx_rx => sub { my $x = $str; $x =~ /.*X/g and $x =~ s/\G.*// },

  # reverse, remove up to first X, reverse
  sexeger => sub {
    my $x = $str; ($x = reverse $x) =~ s/^[^X]+//; $x = reverse $x;
  },

  # find the last X, remove everything after it
  substr => sub { my $x = $str; substr($x, rindex($x, "X")+1) = "" },
});

__END__
               Rate      last   sexeger capt_repl     rx_rx    substr
last        6398/s        --      -84%      -84%      -90%      -97%
sexeger    39625/s      519%        --       -4%      -37%      -83%
capt_repl  41274/s      545%        4%        --      -35%      -82%
rx_rx      63380/s      891%       60%       54%        --      -73%
substr    230796/s     3507%      482%      459%      264%        --