Need dependency-free way of capturing verbose function output in a variable

As part of the work which Richard Elberger and I are doing for 
File-Path, we would like to reduce the test suite's dependency on 
libraries not distributed with the Perl 5 core.  Specifically, that 
means eliminating use of Test::Output functions stdout_is(), stderr_is() 
and stderr_like().

I know how to use $SIG{__WARN__} and $SIG{__DIE__} to replace the last 
two functions.  That leaves only the problem of capturing STDOUT when a 
user has requested verbose output from mkpath, make_path, rmtree or 
remove_tree.

If we were not working on a library distributed with core, I would use 
any one of IO::Capture, IO::CaptureOutput or Capture::Tiny.  (I've even 
contributed to some of them!)  But even Capture::Tiny (a) is not (yet) 
core; and (b) has a dependency on File::Temp, which in turn has a 
dependency on File::Path.

ISTR that there's some magic you can perform with 'select' to obtain 
this objective, but I can't seem to locate what I have in mind on the 
network.  What I want is something that, with just core Perl will be 
callable like this:

     my $captured_verbose_output =
         _run_for_verbose( sub {@created = mkpath($dir, 1)} );
     is(
         $captured_verbose_output,
         "mkdir $base\nmkdir $dir\n",
         'mkpath verbose (old style 1)'
     );

... where _run_for_verbose() has this look:

     sub _run_for_verbose {
         my $code = shift;
         my $captured_verbose_output;
         ... # filehandle magic; no non-core libraries
         &$code;
         ... # complete the magic
         return $captured_verbose_output;
     }

Suggestions?  Thanks in advance.

Jim Keenan