Your recursive version of max is broken for lists like (-1,0,1,2) and (-1,undef,1,2). i.e. the code assumes it is at the end of the list whenever $next==0 or $next==undef (which isn't true in general). Here's a snazzy (if not the most efficient) recursive version with a hat tip to Zaxo...

sub max {
    my ($x, @xs) = @_;
    @xs ? ($x, max(@xs))[$x < max(@xs)] : $x
}
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