From "The Pragmatic Programmer" (tip #22):

DRY - Don't Repeat Yourself - Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

In short, when in doubt, make a sub. Name the sub properly, and you have replace several lines of code in the original location with a sub call that is, in effect, a comment.

As to sub size, someone suggested no more than one page. I follow the rule that a sub (including comments and white-space) must fit on my screen without scrolling. Only excpeption is if there is a long list of constants or other literal text.

I find it much easier to have subs like:

sub foo {
    readstuff();
    formatstuff();
    writestuff() if check_stuff();
}
[download]

I have seen some code that reads like English until you get to the subs that actually do anything. And the subs that do anything are just a few lines long, easy to understand, and documented by the sub name (check_and_exit_if_bad()).

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