$ 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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

$ 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

##</code><code>##

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