All right except demerphq got it the wrong way round.
What he meant was:

unless ( caller ) {
    # here go the tests that are only called when the module is run as
+:
    # perl Module.pm
}
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It's part of my cargo cult programming style. You know the documentation of caller, as you've already written in this thread.

Cheers, Sören

Update: I just saw a similar thing on a thread at German language Perl-Community.de.
eserte writes:

return 1 if caller; # end of the module's own code

# Test Code goes here
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