Probably what you wanted is "my @newa;" instead of "our @newa;". "our" is a special thing usually seen in a module. Read perl doc our for more info. In a module, a my variable cannot be exported, but an our variable can be.

Yes, grep is the right tool here. I personally prefer the block form of grep and map.

my @a = 1..1000; 
my @newa = grep{$_>850}@a;
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Grep is a "filter" operation.