:~$ perl -wde0
...
  DB<1> my %a; $a{1}=$a{1}+1;print $a{1}
Use of uninitialized value $a{"1"} in addition (+) at (eval 5)[/usr/share/perl/5.10/perl5db.pl:638] line 2.
...
1
  DB<2> my %a; $a{1}++;print $a{1}
1