my @a = qw( a x z );
{
  local $" = "";
  while (<>) {
    print if /^[@a]{3}$/;  # /o might be good, too
  }
}

##</code><code>##

my @a = qw( a x z );
my $re = join '', @a;
while (<>) {
  print if /^$re$/;  # again, /o might help
}