$ perl -de 42

Loading DB routines from perl5db.pl version 1.32
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(-e:1):   42
  DB<1> %hash = qw( a 1 b 2 c 3 d 4 );

  DB<2> print %hash
c3a1b2d4
  DB<3> print scalar %hash
4/8
  DB<4> $hash{e} = undef

  DB<5> print scalar %hash
5/8
  DB<6> print "true" if defined $hash{e}

  DB<7> print "true" if exists $hash{e}
true
  DB<8>