Its partly documented in perlvar
but mostly in the camel. Basically all the digit scalars are used for doing pattern match storage. Apparently one of the other scripting langugae had a limit (i think 9) on the number of capture buffers allowed. I believe it was a matter of pride that perl could handle an unbounded number (ykwim). And as all of the capture buffers are read-only that makes them all read only. The lexical bit I dont remember. :-)
my friends call me, usually because I'm late....