This does an in-place edit of the underlying hash. With your $W above, you could flatten level 0 or 1. Flattening level 2 leaves it unchanged. Having other reference types intermingled will induce barfing.

sub do_for_level {
   my ($href, $level, $code) = @_;
   if ($level == 0) {
      %$href = map $code->($_, $href->{$_}), keys %$href if ref $href;
   }
   else {
      map do_for_level($_, $level-1, $code), values %$href;
   }
}

sub flatten_level {
   my ($key, $href) = @_;
   ref $href ? map {$_ => $href->{$_}} keys %$href : ($key, $href);
}

use Data::Dumper;
do_for_level($W, 2, \&flatten_level);
print Dumper($W);
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