Method::Signatures provides two new keywords, func and method, so that you can
write subroutines with signatures instead of having to spell out my $self =
shift; my($thing) = @_

func is like sub but takes a signature where the prototype would normally go.
This takes the place of my($foo, $bar) = @_ and does a whole lot more.

method is like func but specifically for making methods. It will automatically
provide the invocant as $self (by default). No more my $self = shift.

It also allows signatures, very similar to Perl 6 signatures.

It also does type checking, understanding all the types that Moose (or Mouse)
would understand.

And it does all this with no source filters.