I think you are just asking for this:

#!/usr/bin/env perl
use strict;
use warnings;

my $json =
[
          {
            'readings' => [
                            {
                              'asddad' => '123',
                              'fsfsf' => 'abc',
                              'sdfsdfsfsf' => 'x1y',
                            }
                          ],
            'FWTYPE' => 'XXX',
            'FWID' => '123'
          }
];

# Keys and values
my $hashref = $json->[0]{readings}[0];
for my $k (keys %$hashref) {
    print "Key '$k' has value '$hashref->{$k}'\n";
}

# Just keys
my @k = keys %$hashref;
print "All keys: @k\n";
[download]

See also perldsc for more on these sorts of nested data structures.