sub ff {
    my ($x, @y, %z) = @_;
    ...
}

Prototypes indicate expected parameter structure.
Use one $ for each scalar and @ for a list.

Examples:
sub f($$$);
sub ff($@%);
or even
sub xyz(\[$@%]);


Perl check against the prototype provided it occurs before
the first call on the function.

Put prototypes at the top of the file.