The biggest expense is going to be if you have a very large @INC that you're searching across. If files are located further down in @INC, such as if you're loading stuff you installed from CPAN, or if some of your @INC directories are on NFS mounts, those are going a much longer time to resolve than any import/export stuff you're dealing with.

As for import/export stuff, I have a policy in my CPAN modules to never import stuff into my namespace, preferring instead to do the following:

use Scalar::Util ();

if ( Scalar::Util::reftype( $thing ) eq 'HASH' ) {
    # Do stuff here
}
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This has a couple benefits (other than performance, which I wasn't thinking of) to my thinking:

• Almost everything of mine is OO. Perl's OO treats all subroutines in a given namespace as methods in that class. I want to keep my classes as clean as possible, particularly if I want to have an AUTOLOAD.
• External subroutines are immediately labelled as to where they come from for future maintainers. While some people aren't as thrilled by this, I have been stymied by automatic exports and where they came from.