That's certainly a valid solution, but of course TMTOWTDI

As for why the code produced the error in the first place, you are starting with an array of N elements, and you are iterating over a list of the indexes of the elements in that array. But while you iterate over those elements, you are deleting them from the orriginal array, which shortens it, but you've still got the indexes, so as you get to the end of your list of indexes, the last few indexes may not exist in the array anymore.

Ie: if $#good_ones is 9, and the camel picks up $good_ones5 and you splice it out of the list, then when that foreach loop reaches 9, $good_ones9, it's going to be undef (because it's off the end of the array)

Another valid solution would be to change your foreach(0..$#good_ones) to for(my $_=0;$_<=$#good_ones;), and only increment $_ if you do NOT splice the array. In that case, $#good_ones will be evaluated on each pass of the loop, and it will do the right thing as the array gets shorter and shorter.