local $np; # I added this declaration myself
$np = qr{
          \{
          (?:
             (?> [^{}]+ )
             |
             (??{ $np })
           )*
           \}
        }x;

##</code><code>##

Use of uninitialized value at shorty.pl line 19 (#1)
    
    (W) An undefined value was used as if it were already defined.  It was
    interpreted as a "" or a 0, but maybe it was a mistake.  To suppress this
    warning assign an initial value to your variables.
    
Uncaught exception from user code:
        /
    {
    (?:
     (?> [^{}]+ )
     |
     (??{  })
     )*
         }
/: Sequence (??...) not recognized

##</code><code>##

Uncaught exception from user code:
        /
    {
    (?:
     (?> [^{}]+ )
     |
     (??{ foo })
     )*
         }
/: Sequence (??...) not recognized