What is /o really for?

Use of /o is irrelevant unless variable interpolation is used in the pattern, and if so, the regexp engine will neither know nor care whether the variables change after the pattern is evaluated the very first time.

/o is often used to gain an extra measure of efficiency by not performing subsequent evaluations when you know it won't matter (because you know the variables won't change), or more rarely, when you don't want the regexp to notice if they do.

For example, here's a ``paragrep'' program:

    $/ = '';  # paragraph mode
    $pat = shift;
    while (<>) {
        print if /$pat/o;
    }