if ($shell ne "" and $shell !~ /false|null/) {
  # it's ok
}

##</code><code>##

if ($shell ne "" and $shell !~ /false/ and $shell !~ /null/) {
  # it's ok
}

##</code><code>##

"foo" =~ /foo/;
print grep //, qw( bar foobar );  # foobar