Just copy AnyEvent/DNS.pm from the AnyEvent distribution somewhere under your source tree (for instance, under a directory called patched_modules), make the required modifications there and then ensure it is in the module search path before the original:

BEGIN { unshift @INC, './patched_modules' }

use AnyEvent;
use AnyEvent::DNS;
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I would also add a version check into my modified version of AnyEvent::DNS:

AnyEvent::VERSION eq "7.17" or
  warn "New version of AnyEvent ($AnyEvent::VERSION) detected, please 
+upgrade ".__FILE__." accordingly";
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