Your arguments were completely wrong. Maybe you should reread the documentation for substr. Corrected:

my $newstring = substr($string, 4, 2)   # Month (was at 4 len 2)
              . substr($string, 6, 2)   # Day   (was at 6 len 2)
              . substr($string, 0, 4);  # Year  (was at 0 len 4)
[download]

which can be simplified to

my $newstring = substr($string, 4, 4)   # M+D  (was at 4 len 4)
              . substr($string, 0, 4);  # Year (was at 0 len 4)
[download]