However, if you do bench it, please do post results.

#!/usr/bin/perl
use warnings;
use strict;

{   package Exported;
    use Time::HiRes qw{ gettimeofday };
    sub foo { gettimeofday }
}

{   package FullName;
    use Time::HiRes;
    sub foo { Time::HiRes::gettimeofday }
}

{   package CodeRef;
    use Time::HiRes;
    *foo = *Time::HiRes::gettimeofday{CODE};
}

use Test::More tests => 2;
cmp_ok abs(Exported::foo() - FullName::foo()), '<', 1e-4, 'almost same
+';
cmp_ok abs(Exported::foo() - CodeRef::foo()),  '<', 1e-4, 'almost same
+';

use Benchmark qw{ cmpthese };
cmpthese(-3, {
    exported => \&Exported::foo,
    fullname => \&FullName::foo,
    coderef  => \&CodeRef::foo,
});
[download]

On my i3 notebook:

	Rate	exported	fullname	coderef
exported	3537621/s	--	-6%	-72%
fullname	3753422/s	6%	--	-70%
coderef	12498685/s	253%	233%	--

Update

    coderef => sub { CodeRef::foo() },
[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]