Why do you want to use prototypes? They almost never do just what you want and/or expect. In any event, optional parameter values must be handled in the same way: by checking in the function that a defined (or otherwise valid) value has been passed.
>perl -wMstrict -le
"sub opt_w_proto ($;$) {
my ($x, $y) = @_;
$y //= 42;
print qq{x $x y $y};
}
;;
sub opt {
my ($x, $y) = @_;
$y //= 42;
print qq{x $x y $y};
}
;;
opt_w_proto(7);
opt_w_proto(7,0);
;;
opt(7);
opt(7,0);
"
x 7 y 42
x 7 y 0
x 7 y 42
x 7 y 0