Well, both  foo and (?-xism:$STUFF) are regular expressions and valid strings. A better way to distinguish the two is to look at the reference:

$reg = qr'foo';
$str = 'foo';
$str2 = '(?-xism:foo)';
$reg_ref = \$reg;
$str_ref = \$str;
$str2_ref = \$str2;
print "$reg -> $reg_ref\n";
print "$str -> $str_ref\n";
print "$str2 -> $str2_ref\n";
[download]

yields

(?-xism:foo) -> REF(0x80635ac)
foo -> SCALAR(0x80635dc)
(?-xism:foo) -> SCALAR(0x8063600)
[download]

-Mark