I agree with perrin on the require portion. However - on the constant portion - I have used schemes similar to the following many times:

use constant foo => do {
 my $foo;
  # do something that is really hard to lookup or calculate
 $foo; # return of the do
};
[download]

This way, say you are in a daemon, or long running process, or mod_perl, and so on - you can get the compile time benefits, even with something that might take a minute or so to calculate the first time. It doesn't fit all situations - but for those it does it is nice to do a Deparse and see chunks of code removed when doing a

if (foo) {
} else {
}
[download]

my @a=qw(random brilliant braindead); print $a[rand(@a)];