Request translation into Perl

I'm testing something, and I'm stuck trying to write a Perl equivalent of this function because Perl's syntax is made of ugly, rudimentary, hack and fail. Could you please show me how to do this very simple thing in Perl?

function f($i = 1, $r = 17) {
    return --$r <= 0 ? null : array($i => f($i + 2, $r), $i + 1 => f($i + 3, $r));
}
Thanks!

I fail to see the point too as it can very easily be computed directly without resorting to atrociously inefficient recursion.

You can't set default function values in perl :
sub f { $_[1] <= 0 ? null : [$_[0] => f($_[0]+2, $_[1]-1), $_[0]+1 => f($_[0]+3, $_[1]-1)]; }