On Debian Wheezy with its outdated packages, the version of Net::SMTP::SSL is 1.01. If you try to use send-email, the script will crash with the following error:
STARTTLS failed! SSL connect attempt failed with unknown error error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed at /usr/libexec/git-core/git-send-email line 1294
Apparently, that is because the script is calling a subroutine (start_SSL()) that is missing in older versions. I guess the script should check the minimum required version before proceeding with the execution.