Not knowing anything about the flow program all I can suggest it that it has a pager flag and in its interaction with Net::Telnet is delivering a virtual screenful. I don't definitively know how to fix flow or the Net::Telnet interaction. Regardless this hack should work around it. If I pipe a stream through more and then to cat:

$ ll | more | cat
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Then more is effectively disabled as cat accepts the stream and just pipes it out so it streams several screenfuls. Certainly worth a try and I can't see why it will not work as we just abstracted the output from flow away from the vtty, but without using a file.

Next suggestion is to try:

$obj->option_accept( Dont => TELOPT_NAOP, Dont => TELOPT_NAWS )

# from the source we see the constants being declared.....
# sub TELOPT_NAOP ()          {9}; # Output Page Size
# sub TELOPT_NAWS ()         {31}; # Negotiate About Window Size
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If you dig into the souce you should find a burried use IO::Pty which does itself have a method to control the window size but no easy way to get at it short of hacking the source.