steve@kirk:~$ perl -wle'open FOO, undef || die "FOO! $!";'
Name "main::FOO" used only once: possible typo at -e line 1.
FOO!  at -e line 1.
steve@kirk:~$ perl -Mstrict -wle'open FOO, undef || die "FOO! $!";'
Name "main::FOO" used only once: possible typo at -e line 1.
FOO! Bad file descriptor at -e line 1.