$ perl -wcT clock.cgi
Name "main::opt_g" used only once: possible typo at clock.cgi line 5.
Name "main::opt_u" used only once: possible typo at clock.cgi line 6.
Name "main::opt_f" used only once: possible typo at clock.cgi line 7.
clock.cgi syntax OK
Pay attention to the warnings, as well. Perl's syntax checker
has really improved over the years, and will alert you of many
possible errors, such as using non existent variables, uninitialized
variables, or file handles.
And finally, if there are no warnings or errors, you will see:
$ perl -wcT clock.cgi
clock.cgi syntax OK