$ cat test.pl
use 5.18.0;
use warnings;

use Benchmark qw( cmpthese );

my $x = join "    " => "", ("abc") x 5, "";

sub splt {
    split " ", reverse ((split " ", (reverse $x), 1)[0]), 1;
    }
sub rgx {
    $x =~ s/^\s+//r =~ s/\s+$//r;
    }
sub trx {
    my $y = $x =~ y/ /x/cr;
    substr ($x, index ($y, "x"), rindex ($y, "x") - length ($x) + 1);
    }

say "sourc: |$x|";
say "split: |", splt (), "|";
say "regex: |", rgx  (), "|";
say "tr/x/: |", trx  (), "|";

cmpthese (-2, { splt => \&splt, rgx => \&rgx, trx => \&trx });