IvanH:

It means that Email::Send::SMTP::Gmail->new() failed somehow and didn't create a valid object for the $mail variable. I don't see anything obvious in the Email::Send::SMTP::Gmail docs about how to do diagnostics, so you may need to look over the source code to see why it may fail, or look at the related classes (e.g. Email::Send::SMTP) for diagnostic information.

If you're using SMTP, you might consider running a telnet session to gmail to see if you can connect to the server and log in properly with the credentials you're using.

...roboticus

When your only tool is a hammer, all problems look like your thumb.