my $qfn = '/tmp/myfile.index';

open(my $fh, '<', $qfn)
   or die("Can't open \"$qfn\": $!\n");

my $index = <$fh>;
++$index;

open($fh, '>', $qfn)
   or die("Can't create \"$qfn\": $!\n");

print($fh "$index\n");