The basic description is found in perlmod:

A "BEGIN" code block is executed as soon as possible, that is, the moment it is completely defined, even before the rest of the containing file (or string) is parsed. You may have multiple "BEGIN" blocks within a file (or eval'ed string) -- they will execute in order of definition. Because a "BEGIN" code block executes immediately, it can pull in definitions of subroutines and such from other files in time to be visible to the rest of the compile and run time. Once a "BEGIN" has run, it is immediately undefined and any code it used is returned to Perl's memory pool.

Also note that use has an implicit BEGIN block around it.

In reply to Re^4: repeated use of module and EXPORT by almut
in thread repeated use of module and EXPORT by rpelak

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