in reply to reading the wrong input file out of 2 opened file

while (<F0>)
reads a line from F0 and assigns it to $_, and stops the loop unless the value is defined.

On the other hand,

while (<F1> && ($line_stop++ < $line_no))
reads a line from F1, but doesn't assign it anywhere, and would stop the loop if the value was false.

See I/O Operators for details.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]