For me, one part laziness and one part flexibility, and the flexibility part I might be rationalizing. :P. Perl's a superset of what can be done with sed, so I tend to use perl for this case even when sed would do, just because I can remember the perl command line args and regex syntax off the top of my head. I often have to take a trip to the sed man page if I use sed. It's easier for me to add special corner cases to a perl one-liner, etc.

That said, sed is simpler, sometimes less to type, sometimes there on systems where perl isn't (though that seems uncommon these days). But I'm using sed more often, and the more I use it the more familiar it is. Nicer to have two good tools around than just one, right?