Установите для переменной среды PERL5OPT значение '-d':
$ export PERL5OPT=-d
$ ./test.pl
Loading DB routines from perl5db.pl version 1.3
Editor support available.
Enter h or `h h' for help, or `man perldebug' for more help.
main::(./test.pl:3): print "Hello\n";
DB<1> r
Hello