If output from external command is truncated, maybe it's this bug (in perl 5.14 ):
https://rt.perl.org/rt3/Ticket/Display.html?id=119097
http://www.perlmonks.org/?node_id=1026468

i.e. data received from pipe can be truncated if SIGnal received (and you have signal handler, thus you are affected). I suggest check $! after backtricks call.