After peeking at the source code, I too noticed the lack of XModem::Transmitter (or anything similar). The following blurb from the comments jumped out at me though:

# Additionally, this is ALPHA software, still needs extensive
# testing and support for generic AT commads, so use it at your own ri
+sk,
# and without ANY warranty! Have fun.
#
# This Xmodem protocol version is indeed very alpha code,
# probably does not work at all, so stay tuned...
#
# $Id: Xmodem.pm,v 1.7 2005/11/15 22:28:42 cosimo Exp $
[download]

It looks as though Cosimo hasn't had the time nor the inclination to work on this lately. You could either try to contact this person and politely inquire. Alternatively, if you like a challenge, you could write your own XModem::Transmitter using the code for XModem::Receiver as a guide.