$ perl -de 1

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

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

main::(-e:1):   1
  DB<1> use bigrat;

  DB<2> print 1/3 + 1/4,"\n";
0.583333333333333

  DB<3> use bigrat; print 1/3 + 1/4,"\n";
7/12

  DB<4> q