The unfortunate truth about "console mode" is that it really isn't the TELNET protocol, just as the docs for Net::Telnet say. There's a reason there are RFC and STD documents, and there's a reason responsible developers try to maintain compatibility with them.

HP, it seems, has not done so in their default configuration. I would suggest that you contact your equipment vendor for a fix to their issue, because that's where the trouble lies. If you get no satisfaction dealing with their nonstandard implementation, then perhaps you can get some software to deal with it. In any case, consider it a lesson learned about standards and shop for hardware that follows them next time if you are fortunate enough to have that sort of input in your organization.

In the meantime, have you tried just setting a delay in your program and using Net::Telnet to send the commands to disable the offending option "in the blind"? If you can issue the authentication, wait, issue the command to disable "console mode", wait, and issue the command to write the config (if there is one and this line doesn't use a persistent object -- I admit I've not used a lot of HP networking equipment), then maybe you can automatically get into a usable TELNET session in which you can reliably read prompts after that.