I'm going to guess that it's one of two things:

• Your prompt setting is not being matched. (Did you mean to use both $user and $username in your code?) You can use the dump_log() method to check for this stuff.
• The literal command backup_sybase.pl is not found. Perhaps you need to give it a path such as ./backup_sybase.pl or maybe /usr/local/something/backup_sybase.pl?

Also, I could very well be wrong, but my take on the docs is that you want the prompt() setting to match the whole prompt, not just a portion of it.

cheers!