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  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.

        The auto-increment operator has a little extra builtin magic t
+o it.
 
...
       "undef" is always treated as numeric, and in particular is chan
+ged to 0
       before incrementing (so that a post-increment of an undef value
+ will
       return 0 rather than "undef").