Looking at the docs for Gearman::Client that do_task method returns an undef on failure or a scalar ref to the result value on success. You need to check for that failure (and/or other possible failures; handwavy sample follows but you could go even further checking if decode_json throws an error) before you try and dereference it to decode whatever JSON into something you can treat as a hashref.

my $result = $client->do_task( ... );
if( not defined $result ) {
  ## Presumably there may be some other information about the failure 
+but I don't
  ## see offhand how to retrieve that; but that's a good thing to incl
+ude in your
  ## error message here
  die "isok not ok for ip '$ip'!\n";
}

my $json_result = decode_json( ${ $result } );
if( not ref $json_result or not ref $json_result->{ $ip } or ref $json
+_result->{$ip} ne q{HASH} ) {
  die "Did not get back the hashref expected for '$ip'\n";
}

my @result = keys %{ $json_result->{ $ip } }
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Edit: Tweaked variable name last line. Derp. And expanded on description of method's return value. MOAR EDITS