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If you forget the 1 at the end of a package, Perl tells you "The package didn't return a true value". Well, if it knows you forgot it, why not just put it there for you?
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1 at the end of a module means that the module returns true to use/require statements. It can be used to tell if module initialization is successful. Otherwise, use/require will fail. $somevar is a variable which is accessable only inside the block. It is used to simulate "static" variables.
Inside an END code block, $? contains the value that the program is going to pass to exit() . You can modify $? to change the exit value of the program.
A Perl package is a collection of code which resides in its own namespace. Perl module is a package defined in a file having the same name as that of the package and having extension . pm. Two different modules may contain a variable or a function of the same name.
Because Perl modules are required to return a value to signal if the require directive must succeed (true value returned) or fail (false value returned; this can make sense if the module failed to initialize for some reason).
If you don't return anything, the interpreter cannot know if the require must succeed or fail; at the same time, since it's easy to forget to put the true value at the end of the package, it suggests the "common fix" for this error: add a true value as a return.
For some other info/folklore about the modules return value have a look at this question.
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A package can return a false value if it fails to initialize, for example if it couldn't find a required data file or external library. This way it fails cleanly at load time (and this failure can even be tested for) rather than unpredictably later.
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