[Perl] better code injection handling for perl client

[wing328]

For #3201

Test cases passed

ok 32 - accept_tokens() correctly set api_key
ok 33 - accept_tokens() correctly set api_key_prefix
ok 34 - accept_tokens() correctly set api_key_in
ok 35 - clear_tokens() returns the correct data structure
ok 36 - bad token causes exception
ok 37 - no (unexpected) warnings (via END block)
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 12.514 s
[INFO] Finished at: 2016-06-28T16:35:42+08:00
[INFO] Final Memory: 11M/245M
[INFO] ------------------------------------------------------------------------