How can I test a standalone Perl script?

It depends on whether you want to test the script itself, or test the subs that make up the script. If you want to test the script, then an external test would be more appropriate, e.g. a shell script. If you want to test the functions that make up the script, then you can either write those tests as more functions within the script or refactor the elements into a Perl module and test the module (which you say you don't want to do).

If the script is small enough, then refactoring might not be necessary. Simply add a '-test' command line argument and call into a test sub, which in turn tests everything else. If doing this, I like to print out a progress indicator of some kind (e.g. a '.' for every test that passes).

If the script is more complex, though, you may want to consider refactoring bits into one or more modules and testing them with Test::Simple or Test::More.