I understand your nervousness about extra arguments, but I've learned that they don't really matter in Perl. An analogy that comes to mind is filling a glass of water. If I hand you a cup and you pour a gallon of water into it, all I get back is a cup. I might make fun of you for being so sloppy, but who cares? After all, I have the cup of water that I wanted.

Still, if it is important to you, you can enforce it with logic in your sub.

sub xyz {
    my $xxx = shift || croak('Missing xxx parameter');
    my $yyy = shift || croak('Missing yyy parameter');
    my $ttt = shift;

    # Enforce length of function parameter list.
    if (@_) {
        croak('Too many args for xyz!');
    }

    # ...
}
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... but things like that risk making the code a little harder to read.