There are only three numbers in Computer Science. Zero, One and N
Someone suggested on the london.pm mailing list some time ago that if you ever use a constant other than 0 or 1 you should comment why. I can see his point, although I make exceptions for well-known scaling factors like 7, 24, 60, 3600 and 86400 when mangling times and so on. I also don't bother documenting the constants when I do something like:
$foo = ($foo & 112) >> 4;
although that gets wrapped up in a little subroutine whose function is documented, including a pointer to the relevant standard. But just about every other constant I do inded comment. eg, from my current project ...
$price /= 100; # convert from pence to pounds
$time += 23 * 3600 # because we're changing timezone