As far as I understand the JSON syntax, your document contains only one array (under 'dev' key) which consists of only one hash (with only one key called 'nfsmount'). All other data structures are hashes, which only allow unique key names.

Thus, you need to print only two hash keys and there is no need in manual listing of all keys in this document. Try this:

use JSON;
use Data::Dumper;
...; # open files, read the JSON string
my $data = decode_json $your_json_string;
for (qw/bigstor_nfs test_nfs/) { # list of keys to search for
 print {$filehandle} Dumper { $_ => $data->{tag}{dev}[0]{nfsmount}{$_}
+ } if exists $data->{tag}{dev}[0]{nfsmount}{$_};
 # print to prevousely opened filehandle:
 # the results of "dumping" of anonymous hash reference
 # containing the key and its value
 # if this key exists in the main hash
}
[download]