The best value for Wufniks $ff (the fuzz factor or often epsilon) is given in POSIX. From Mastering Algorithms with Perl (the wolf book pp.472 - 474)

If you have the POSIX module you may use the DBL_EPSILON constant that it defines:

use POSIX;
use constant epsilon => 100 * DBL_EPSILON;
[download]

Very useful in dsp to write machine independent code. You dont't want to find it all breaks when you port it somewhere else. Although its messier in just one place Abigails choice of overloading the operator is probably the most elegant general solution imo.