use strict;
use warnings;
use Time::HiRes 'time';

my $str;
$str .= chr 97 + rand 2 for 1 .. 5e3;

{
my $n = 0;
my $s = $str;
my $t = time;
$n ++ while $s =~ s/((.)(?1)?\2)//g;
printf qq(%3d loops, %.3f s, result: "%s"\n), $n, time - $t, $s
}

{
my $n = 0;
my $s = $str;
my $t = time;
$n ++ while $s =~ s/(.)\1//g;
printf qq(%3d loops, %.3f s, result: "%s"\n), $n, time - $t, $s
}

#  4 loops, 1.542 s, result: "babababa"
# 47 loops, 0.001 s, result: "babababa"