$# was a variable that could be used to format printed numbers. After a deprecation cycle, its magic was removed in Perl 5.10 and using it now triggers a warning:
$# is no longer supported.
This is not the sigil you use in front of an array name to get the last index, like
$#array. That's still how you get the last index of an array in Perl. The two have nothing to do with each other.
Deprecated in Perl 5.
Removed in Perl 5.10.