Please allow to specify STASH_HOOK_ADDRESS via bitbucket.properties file or via environment variable. How it call ServerSocket to get local host address. And if it return IPv6 address Perl hook fail on line:

my $socket = new IO::Socket::INET

It should be replaced to:

my $socket = new IO::Socket::INET6

(also added use IO::Socket::INET6; at top of hook-callback.pl)

The trouble is in that this file autogenerated on each bitbucket restart, and you should make changed after each restart.