I don't believe that the overhead is in exporting the functions, but in the ability for the methods to "swing both ways".

Every dual-nature (ie. functional and OO) method in CGI starts off by calling an internal sub, self_or_default, to determine the calling style. If the method was called as an export, self_or_default has to do the additional task of unshifting the default CGI object ($Q in the CGI.pm source) onto @_. This negates any performance gain from not having the method-call overhead:

use strict;
use warnings;
use Benchmark qw(cmpthese);

use CGI qw(:standard);
my $q = new CGI;

cmpthese (-3, {
  method => sub { $q->header; },
  direct => sub { header; }
});

### RESULTS ####

          Rate direct method
direct 14483/s     --    -4%
method 15106/s     4%     --
[download]

So it appears as if OO calling style on CGI is indeed faster in practice.

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print