package Foo;

# some code here

package main;

return 1 if caller();
print "as a program\n";

##</code><code>##

$ perl Foo.pm
Called as a program!

##</code><code>##

$ perl -MFoo -e 1
$ 

##</code><code>##

$ perl Bar.pm
Can't open perl script "Bar.pm": No such file or directory