Fun Perl question

[Peter Corlett]

So there's me with this bit of code that's running a bit slow. I run the
following command:

  perl -M:DProf ./test.pl

and then follow it with dproffpp to see the results. I'm then left
scratching my head to work out why I've got "Unable to open tmon.out" before
spotting my silly mistake.

The fun question is: why did the original command seem to work?

(I've already worked out the answer, by the way, but it might be an amusing
puzzle for somebody else.)