From perldoc Exporter:

    
       If the first entry in an import list begins with !, : or /
       then the list is treated as a series of specifications
       which either add to or delete from the list of names to
       import. They are processed left to right.
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So it is documented but probably not very clearly. On the other hand I fail to see why it should be this way. IMHO it is not Perlish (i.e. it is no DWIM). I've been biten by it myself. Once I spent about an hour trying to understand what the hell is going on and why I can't get Exporter work when I placed :foobar at the end of the import list.

--
Ilya Martynov (http://martynov.org/)