use strict;
use warnings;

use Data::Dumper;

my %hash = (
    a => 1,
    n => 1,
    z => 1,
);

my @ary = qw( a b n m y z );

for my $key ( keys %hash ) {
    for my $elem (@ary) {
        next if not defined $elem;
        if ( $elem =~ $key ) {
            $elem = undef;
        }
    }
}

print Dumper \@ary;

##</code><code>##

$VAR1 = [
          undef,
          'b',
          undef,
          'm',
          'y',
          undef
        ];