#!/usr/bin/perl -l

# http://perlmonks.org/?node_id=1171353

use strict;
use warnings;

my $have = 'A B _ B A A A';
my $remove = 'A B A';
my @left;
my %left;

my $pattern = $remove =~ s/ /(.*)/gr;

$have =~ /^$pattern$(?{ push @left, "$`$1$2$'" })(*FAIL)/;

s/  +/ /g, s/^ //, s/ $//, $left{$_}++ for @left;

print for sort keys %left;