You can also use a queue.

All off-hand code.

my @q[0..1] = ( "\n" x 2);  # other initialization may be apropos

while (1) {
   @q[0..1] = ($q[1], scalar( <INPUT>));
   last if not defined $q[1];
   # previous line is $q[0]
   # currentline is $q[1]
   do_something(\@q);
}
# current line, the last line is now in $q[0]
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Something like this may be used to empty the queue inside the loop. Replace the line with "last" with this:

# handle last line in loop
    if ( not defined $q[1] ) {
         $q[1] = $q[0];
         $q[0] = "\n";
          do_something( \@q);
          last;
     }
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