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
}