DB<1> print $a
 
  DB<2> use warnings; print $a
Use of uninitialized value $a in print at (eval 6)[/usr/share/perl/5.10/perl5db.pl:638] line 2.
..
  DB<3> use warnings; $a=undef;print $a
Use of uninitialized value $a in print at (eval 9)[/usr/share/perl/5.10/perl5db.pl:638] line 2.
..
   DB<4> print $a++
0