my %big_hash = (...);
my @to_keep = ('abc', 'xyz');

%big_hash = map { exists($big_hash{$_}) ? ($_ => $big_hash{$_}) : () } @to_keep;
# or
%big_hash = map { $_ => $big_hash{$_} } grep { exists $big_hash{$_} } @to_keep;