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!

Wow I didn't know you fags didn't perl. Now I can't trust any of you when you say it sucks simply because you are obviously ignorant

sub df { return defined($_[0])?$_[0]:$_[1]; }
sub f {
    my ($i,$r) = (df($_[0],1),df($_[1],17));
    return --$r <= 0 ? undef : { $i => f($i + 2, $r), ($i + 1) => f($i + 3, $r) };
}

or

sub f {
    my $i = defined($_[0])?$_[0]:1;
    my $r = defined($_[1])?$_[1]:17;
    return --$r <= 0 ? undef : { $i => f($i + 2, $r), ($i + 1) => f($i + 3, $r) };
}

Your PHP function is quite ugly

sub f {
    my ($i,$r) = (df($_[0],1),df($_[1],17));
    if (--$r <= 0) { return undef; }
    return { $i => f($i + 2, $r), ($i + 1) => f($i + 3, $r) };
}