Perl driver is very, very, very slow compared to PHP one...
Doing some tests with perl 5.10.1 and perl 5.12.3 show that PHP is between 2.8 to more than 3.2 times faster for basic insertions (without or with threads enabled).
In PHP case, mongod reach 100% and PHP stay around 57% CPU. 10M insertions done in ~100 seconds.
In Perl case, mongodb stay around 76% while Perl is at 100% CPU ! 10M insertions done in ~280 seconds (threaded perl) or ~320 seconds (original, debian threaded perl).
I join the 2 sample scripts and a little patch that avoid calling sprintf() 12 times for each insertion. In the test case, 10% of exec time was spent in sprintf. With it we gain 10% of total time, not much, but better than nothing...