you could use Set::Infinite or silimar that can do set logic ... pseduocode:

@lists =(  (12..17), (25..42), (44..50)  );
@gap_fillers = (m..n) - intersection( (m..n), @lists );
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update can probably just do this:

$lists = Set::Infinite->new([12,17], [25,42], [44,50]);
print Set::Infinite->new([m,n])->minus($lists);
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