Oh, and also: if you only want to import a sub into main::, you should not import the whole typeglob. Also, there is probably no need to import into main:: — timestamp is a rather generic name that may be used in some function/module you call, and things will soon get messy. Besides, local would be pretty much useless inside a BEGIN block (and that is very desirable). Use

BEGIN {
    *Module::Derived::timestamp = \&Module::Base::timestamp;
}
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or something.

All this means that symbol table tricks should be severely incapsulated so that no living soul ever sees them Exporter.pm can help remove this mess!