for completeness:

> Of course, that doesn't test if the metachars make sense as a regex

let's Perl parse Perl =)

$ perl -ple ' $_ = eval "sub { /$_/ }" ? "OK\n" : "$@" '
a
OK

a(
Unmatched ( in regex; marked by <-- HERE in m/a( <-- HERE / at (eval 2
+) line 1, <> line 2.

a[
Unmatched [ in regex; marked by <-- HERE in m/a[ <-- HERE / at (eval 3
+) line 1, <> line 3.

a()
OK
[download]

UPDATE

unfortunately not always correct:

$ perl -ple ' $_ = eval "sub { /$_/ }" ? "OK\n" : "$@" '
a[$]
OK
[download]

but

$ perl -e ' "abc" =~ /a[$]/ '
Unmatched [ in regex; marked by <-- HERE in m/a[ <-- HERE 5.010000/ at
+ -e line 1.
[download]

UPDATE

interesting, this happens because it's a run-time error ... (why?)

$ perl -e 'sub {"aaa" =~ /a[$]/}'
$ perl -e '"aaa" =~ /a[$]/'
Unmatched [ in regex; marked by <-- HERE in m/a[ <-- HERE 5.010000/ at
+ -e line 1.
$ perl -ce '"aaa" =~ /a[$]/'
-e syntax OK
[download]

looks like a parser problem for me!

UPDATE

variable interpolation is part of the problem, a[$] is a valid pattern, as long as it's not interpolated:

perl -E 'say q(a$) =~ q(a[$]) '
1
[download]

UPDATE

this is very reliable

perl -ple ' $_ = eval "sub { 'a' =~ q\0$_\0 }" ? "OK\n" : "$@" '

to avoid problems with \0-delim consider using here-docs instead, or even a pack/unpack combination