I'm sorry, but I'm not really following. Maybe you can clarify a bit what's happening 'there' (at the telnetted end) and what's happening 'here' - you seem to have Perl code on both sides and I'm rather confused about what's what.

I'd assume that the remote end would hand over a text line and than get it back, maybe changed - but I'm not at all sure that I got it right.

Maybe you can first solve the problem locally - working directly on a file, maybe using some helper subroutines like 'get_a_line' and 'put_a_line' - and then change the program to give get_a_line and put_a_line telnetting power.

If telnet is necessary at all; I suppose that depends very much on what your powers are 'here' and what your powers are 'there', but overall the whole thing looks a bit weird.