Now, don't get me wrong - what you originally wanted can be done, but do you really want to go there?

use IO::Scalar;
use Test::More qw(no_plan);

my $string;
tie *STDOUT, 'IO::Scalar', \$string;

ok(test_foo() eq 'bar');
ok($string eq 'foo');
$string = '';

ok(test_bar() eq 'baz');
ok($string eq 'bar');
$string = '';

sub test_foo {
   print 'foo';
   return 'bar';
}

sub test_bar {
   print 'bar';
   return 'baz';
}
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)