use File::ReadBackwards qw( );

my $fh_in = File::ReadBackwards->new($fname_in)
   or die("Unable to open \"$fname_in\": $!\n");

open(my $fh_out, '>', $fname_out)
   or die("Unable to create \"$fname_out\": $!\n");

while (defined(my $line = $fh_in->readline())) {
   print $fh_out $line;
}