use Module::Base qw(timestamp);
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is functionally equivalent to

BEGIN { 
    require Module::Base; 
    Module::Base->import('timestamp'); 
}
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So it's at the very least wrapped in a BEGIN block (that is The Thing To Do when using your approach, anyway). It may actually do anything depending on how Module::Base::import was defined.

It does what you mean when it was created by Exporter and timestamp is in @EXPORT_OK.