$ perl -d foo.pl

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::(foo.pl:7):	print "Hello world line 7\n";
  DB<1> a 9 print "action print\n";                                                                                                   
  DB<2> n                                                                                                                             
Hello world line 7
main::(foo.pl:8):	print "Hello world line 8\n";
  DB<2> n                                                                                                                             
Hello world line 8
main::(foo.pl:9):	print "Hello world line 9\n";
action print
  DB<2> n                                                                                                                             
Hello world line 9
main::(foo.pl:10):	print "Hello world line 10\n";
action print
  DB<2> n                                                                                                                             
Hello world line 10
main::(foo.pl:11):	print "Hello world line 11\n";
action print
  DB<2> n                                                                                                                             
Hello world line 11
main::(foo.pl:12):	print "Hello world line 12\n";
action print
  DB<2> n                                                                                                                             
Hello world line 12
main::(foo.pl:13):	print "Hello world line 13\n";
action print

DB<2> 
A 10                                                                                                                          
  DB<3> n                                                                                                                             
Hello world line 13
main::(foo.pl:14):	print "Hello world line 14\n";
action print
  DB<3> n                                                                                                                             
Hello world line 14
main::(foo.pl:15):	print "Hello world line 15\n";
action print
  DB<3>
A *                                                                                                                           
Deleting all actions...
  DB<4> n                                                                                                                             
Hello world line 15
main::(foo.pl:16):	print "Hello world line 16\n";
action print
  DB<4> n                                                                                                                             
Hello world line 16
main::(foo.pl:17):	print "Hello world line 17\n";
action print
  DB<4> n                                                                                                                             
Hello world line 17
main::(foo.pl:18):	print "Hello world line 18\n";
action print