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$str =~ s/X[^X]*$/X/;

$str =~ s/(.*X).*/$1/;

$str =~ s/.*X\K.*//;

$str =~ /.*X/g and $str =~ s/\G.*//;

($str = reverse $str) =~ s/^[^X]+//;
$str = reverse $str;

substr($str, rindex($str, "X")+1) = "";

use Benchmark 'cmpthese';
my $str = "alphabet X alphabet" x 100 . "junk at the end" x 10;
...
capt_repl  41274/s      545%        4%        --      -35%      -82%
rx_rx      63380/s      891%       60%       54%        --      -73%
substr    230796/s     3507%      482%      459%      264%        --

cmpthese(-5, {
  last => sub { my $x = $str; $x =~ s/([A-Z])[^A-Z]*$/$1/ },
...
capt_repl 30599/s      447%        --      -21%      -24%
rx_rx     38948/s      597%       27%        --       -3%
sexeger   40049/s      616%       31%        3%        --

/* C code */
printf("%s", strpbrk("breakfast", "aeiou"));
/* prints: eakfast */

if ("breakfast" =~ /[aeiou]/) {
  $x = substr("breakfast", $-[0]);
}

# crindex is char-class-based rindex()
use String::Index 'crindex';
...
rx_rx     38194/s      584%       25%        --       -4%      -30%
sexeger   39594/s      609%       29%        4%        --      -28%
crindex   54829/s      882%       79%       44%       38%        --