DB<33> my $y;$y++
  
  DB<34> my $y;$y+1
Use of uninitialized value $y in addition (+) at (eval 38)[/usr/share/perl/5.10/perl5db.pl:638] line 2.

##</code><code>##

        The auto-increment operator has a little extra builtin magic to it.
 
...(<i>special magic for incrementing strings, ie <c>++($x='x9') eq 'y0'</i>)...
 
       "undef" is always treated as numeric, and in particular is changed to 0
       before incrementing (so that a post-increment of an undef value will
       return 0 rather than "undef").