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 threads;

  DB<2> threads->new(sub{})->join;
Scalars leaked: 1

  DB<3>