Untested: 
my @keep = qw [two three];
my %keep;
@keep{@keep} = @hash{@keep};
map {$hash{$_} = 0} for keys %hash;
@hash{@keep} = @keep{@keep};

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In reply to Re: performing operation on elements not in list? by JavaFan
in thread performing operation on elements not in list? by Anonymous Monk

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