$ perl -de 42

Loading DB routines from perl5db.pl version 1.33
Editor support available.

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

main::(-e:1):   42
  DB<1> $a //=5;

  DB<2> p $a
5
  DB<3> $a //=6;

  DB<4> p $a
5
  DB<5>