Ah, thank you aitap.

That made sense -- I have gone ahead and used the following code to solve my problem:

my $winsize = pack('SSSS', 25, 80, 0, 0); # rows, cols, #pixelsX, #pixelsY ioctl($exp->slave(), &IO::Tty::Constant::TIOCSWINSZ, $winsize);

As you had suggested, I am no longer referencing a terminal session that is unavailable to me, but instead, am creating one on the fly with the dimensions required. Thanks again for your helpful advice to lead me onto the right track. In the event that you feel there is a better solution please do let me know.