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my $source = {
    f1 => 'garbage',
...
    f10 => [ 'important', 'data' ],
    f11 => [ 'more', 'garbage' ]
};

my $filter = {
    f3 => 1,
...
    },
    f10 => 1
};

my $output = {
    f3 => 'important data',
...
    },
    f10 => [ 'important', 'data' ],
};

sub hash_filter {
  my $source = shift;
...

  return \%output;
}