Perlite is a
microstructure found in some
steels within a certain range of
compositions. But before I dive headlong into the
guts of the definition, I will give a
brief metallurgy primer to get things underway.
For those that don't know, a steel is basically iron with carbon in it. You might think that steels can only vary in one way, therefore: more "carbonny" or less "carbonny". You would be wrong in two ways. First of all, carbonny is not a word. Secondly, there can be a great deal of difference between two steels with identical carbon content. This is due to the fact that time is a factor in the formation of microstructures in metals: if you cool something quickly (throw it in cold water perhaps) then it has little time to arrange itself before it solidifies.
Metallurgists use a visual construction called a phase diagram to explain this, but it makes things more confusing to the beginner, so I'll resist the temptation and leave that link for the over-eager.
So on with the show... Perlite occurs when a steel with a composition close to the eutectoid composition (0.76% carbon by weight) is cooled slowly. This slow cooling gives the iron and carbon plenty of time to arrange themselves into the most energetically favourable microstructure.
By a magical dice-roll of the rules of Gibbs free energy, it turns out that this structure is a lamellar arrangement of alpha-ferrite (iron with a little carbon dissolved in there) and cementite (iron and carbon in the form Fe3C). Ferrite is a soft, ductile phase; Cementite is a hard, brittle phase. This layered arrangement of the two phases therefore provides a best of both worlds scenario with moderate strength and toughness: a steel with a lot of perlite is something of an all-rounder.
Recognising a perlitic microstructure under a microscope is fairly easy. Simply cut up your sample and polish it to a smooth surface finish (a 1 micron finish should do nicely). Then etch it to show the carbon content and examine it under a microscope. Perlite will usually appear at grain boundaries and will look kind of like photocopies of fingerprints, but less regular in shape.
I have gone into some depth here, but possibly not enough if you are still reading this. My intention with this piece was partly to show you that no matter how nicely I try to present it, the world of metallurgy is exceedingly dull and will always be that way.