I believe the canonical method is to use caller as friedo suggests, though I typically just do something like this (which, though I say "typically" isn't often :):

#!/usr/bin/perl
package Foo;

# ...

unless (caller) {
    # ...  execute this code if we're run stand-alone
} else {
    # ...  do whatever module initialization here.
}
__END__
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You'll note that I didn't end my module with the traditional 1; because that's part of the module initialization. If the initialization succeeded it would return a true value. If you really didn't need to do any of that then leave off the else and just put a 1; after the conditional.