That way you duplicated the transformation code.

Here's a version that uses neither temporaries nor duplicated code:

sub natural_cmp ($a is copy, $b is copy) {
   return [cmp] ($a, ).map: *.subst(/(\d+)/, -> $/{ sprintf( "%s%c%s",
+ 0, $0.chars, $0) }, :g).lc
}
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This uses the reduction meta operator.