$q =~ s/^.*$a//sm;

##</code><code>##

$q =~ s/.*$a//s;

##</code><code>##

#!/usr/bin/env perl 
use strict;
use warnings;
use Test::More tests => 1;

my $q = 'qqqweqwe
asdasdasd
zxczxczxc
tyutyutyi
';
my $a = 'zxczxczxc';
my $want = "\ntyutyutyi\n";

$q =~ s/.*$a//s;
is ($q, $want, "With /s only and no anchor - matched");