It didn't work. Everything seemed to install correctly:

$ sh Configure -de -Dusethreads

....
....
Updating GNUmakefile...
Now you must run 'make'.

If you compile perl5 on a different machine or from a different object
directory, copy the Policy.sh file from this object directory to the
new one before you run Configure -- this will help you with most of
the policy defaults.

$ make
...
...
        CCCMD =  cc -c -fno-common -DPERL_DARWIN -no-cpp-precomp -
fno-strict-aliasing -pipe -I/usr/local/include -O3
cc -o a2p -L/usr/local/lib hash.o  str.o util.o walk.o a2p.o -ldl -lm 
+-lc

Everything is up to date. Type 'make test' to run test suite.

$ make test
...
...
All tests successful.
u=3.32  s=3.11  cu=351.85  cs=77.27  scripts=1611  tests=226809

$ sudo make install
...
...
 /usr/local/share/man/man1/cpanp.1
  /usr/local/share/man/man1/cpan2dist.1
  /usr/local/share/man/man1/shasum.1
  /usr/local/share/man/man1/splain.1
  /usr/local/share/man/man1/xsubpp.1
  /usr/local/share/man/man1/a2p.1
  /usr/local/share/man/man1/find2perl.1
  /usr/local/share/man/man1/s2p.1
  /usr/local/share/man/man1/psed.1

$
[download]

But, I get the same results:

$ cat 3perl.pl
use strict;
use warnings;
use 5.010;
use Config;

$Config{useithreads} or die('Recompile Perl with threads to run this p
+rogram.');

$ perl 3perl.pl
Recompile Perl with threads to run this program. at 3perl.pl line 6.
[download]

Do ithreads work on macs?