karls-mac-mini:~ karl$ perl -d -E 1

Loading DB routines from perl5db.pl version 1.44
Editor support available.

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

main::(-e:1):	1
  DB<1> $a=2;

  DB<2> $b=$a + $a;

  DB<3> say $b;
4