none

A Perl function. Logically the negation of any. Returns a true value if no item in LIST meets the criterion given through BLOCK.Sets $_ for each item in LIST in turn:

print "No non-negative values"
  if none { $_ >= 0 } ($x, $y, $z);

Thus, none_u(@list) is equivalent to @list ? none(@list) : undef.For an empty LIST, none returns true (i.e. no values failed the condition) and none_u returns undef.

Note: because Perl treats undef as false, you must check the return value of none_u with defined or you will get the opposite result of what you expect.

https://metacpan.org/pod/List::AllUtils#none-BLOCK-LIST