The match expression can be

    * a scalar
    * a regular expression
    * a code reference
    * undef

When the match expression is a code ref it will be passed each element in the data structure in turn and should return true or false.

  my $iter = diter $data, sub {
    my $v = shift;
    defined $v && !ref $v && $v % 2 == 1;
  };

  while ( defined ( my $path = $iter->() ) ) {
    print "$path\n";
  }

Note that the match code will see all of the elements in the data structure - not just the scalars.

If the match expression is undef it will match those elements whose value is also undef.