I did it with a hash slice. Probably not very memory efficient:

@hash{ grep( !defined($hash{$_}), @mips) } = (999) x @mips;
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Update: Better hash slice solution:

@new{@mips}=map(($hash{$_}||999),@mips);
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I had also missed the requirement to eliminate key/values not in @mips. ++Vavoom.

Impossible Robot