As other monks had pointed out, use lexical scope (my over our). and grep is definitely suitable here. I'm not exactly sure why you need even grep/filtering if you want to find numbers between 850 and 1000? You could as well use.

 my $lower_bound = 850;
 my $upper_bound = 1000;
 my @newa = ($lower_bound+1 .. $upper_bound);
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