They're short pithy rhymed poems, and were originally created by a Dane called Piet Hein who was a scientist, town planner, writer etc... (can't be certain, but I think Piet Hein would have liked Perl).

Though I've not yet killed myself looking (local bookshop, Amazon, bol.com), it looks like his collections of grooks ("Grooks", "More Grooks" and "Still More Grooks") are out of print.
But you can find quite a lot of them online (www.google.com gives quite a good set of results).

Each Grook was illustrated with a simple ink sketch: some of these are very good and add a lot to the poems...

...which is possibly more than can be said for these Perl translations... (posted in Perl Poetry...)

Cheerio!
Osfameron