$ cat t3.pl
#!/usr/bin/perl

use warnings;
use strict;

sub main { warn "called main" }
main() unless caller();

$ perl -e ' do "t3.pl" '
$ perl -e ' require "t3.pl" '
$ ./t3.pl
called main at ./t3.pl line 6.

$ perl -d t3.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::(t3.pl:7):        main() unless caller();
  DB<1> c
called main at t3.pl line 6.
 at t3.pl line 6
        main::main() called at t3.pl line 7
Debugged program terminated.  Use q to quit or R to restart,
  use o inhibit_exit to avoid stopping after program termination,
  h q, h R or h o to get additional info.  
  DB<1> q
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