shark$ perl -d ./fork.pl 

Loading DB routines from perl5db.pl version 1.22
Editor support available.

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

main::(./fork.pl:5):    $|=1;
  DB<1> sub break {}

  DB<2> $SIG{USR1}=\&break

  DB<3> b break
  DB<4> c