I don't know my language archaeology but what about:
- Taint mode.
- tie - alternative implementation of builtin types.
- floating point version numbers (ok, not so good).
- pick your delimiters qq//, q//, qw//, qx//, m//, s///...
- sigils, not just for evaluating/interpolating but
as a different namespace.
I agree with the posts above, that Perl's value is in
providing a rich choice of features from diverse
languages and combining them in a way to easily get
things done.
I think Perl6 will probably rate as truly innovative,
in a number of ways. Grammar/regex integration and
junctions are my picks.