use warnings;
use strict;
use re 'debug';

my $re = qr/(.)\1*/;
my @matches; 
my $string = "xx556xx";

@matches = $string =~ m/($re)/g;
print "@matches\n\n"; # x x x x 5 5 5 5 6 6 x x x x

@matches = $string =~ m/((??{$re}))/g;
print "@matches\n\n"; # xx 55 6 xx