Several options:

• Interrupt the test at that point. The harness should catch it.
• Run the test manually (perl t/pty.t) and capture the output. It may give you a better idea of what's going on.
• If that's the only test that fails, install it anyway.
• Send any failure messages to the author.
• Optionally, fix the error yourself (by looking in the test), and send the author a patch. :)

If you can swing the last option, do it. Otherwise, it's probably just an unportable assumption on the author's part. (Getting terminal stuff right across platforms is tough, though SCO should be a lot closer to POSIX than either Win32 or MacOS.)