Is it safe to read regular expressions from a file?

(?{code})
(??{code})
${code}
@{code}

eval {
    qr/$re/;
};
if ( $@  ) {
    # do something
}

(?: ( ?{1,2} {  # '(' followed by '?' or '??', and then '{'
|   @ { s* [   # a dereference of a literal array, which may be arbitrary code.
)

(?{ code })