A hash value for a key that doesn't exist is referred to as being "undefined". Not surprisingly, the value returned by $hash{"non-existent"} is undef. It's just that undef+1 is 1.

xoxo,
Andy

%_=split/;/,".;;n;u;e;ot;t;her;c; ".   #   Andy Lester
'Perl ;@; a;a;j;m;er;y;t;p;n;d;s;o;'.  #   http://petdance.com
"hack";print map delete$_{$_},split//,q<   andy@petdance.com   >