You're creating a closure on $i (with your callback), but then you change $i later in the loop. What you need to do is create a closure on something that will stay locally with that individual closure:

for my $index (0..$#parts) {
  my $i = $index; # unique per loop iteration
 ... rest of your loop here ...
}
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This causes a unique $i per iteration, and thus will be closed properly with each callback.