$# was a variable that could be used to format printed numbers. After a deprecation cycle, its magic was removed in Perl v5.10.0 and using it now triggers a compilation error: $# 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. (See "Arrays" in perlintro.) The two have nothing to do with each other.
Deprecated in Perl 5.
Removed in Perl v5.10.0.