Chapter 9. Subroutines

If you have a procedure with ten parameters, you probably missed some

—Alan Perlis

Subroutines are one of the two primary problem-decomposition tools available in Perl, modules being the other. They provide a convenient and familiar way to break a large task down into pieces that are small enough to understand, concise enough to implement, focused enough to test, and simple enough to debug.

In effect, subroutines allow programmers to extend the Perl language, creating useful new behaviours with sensible names. Having written a subroutine, you can immediately forget about its internals, and focus solely on the abstracted process or function it implements.

So the extensive use of subroutines helps to make a program more modular, which in turn makes it more robust and maintainable. Subroutines also make it possible to structure the actions of programs hierarchically, at increasingly high levels of abstraction, which improves the readability of the resulting code.

That's the theory, at least. In practice, there are plenty of ways that using subroutines can make code less robust, buggier, less concise, slower, and harder to understand. The guidelines in this chapter focus on avoiding those outcomes.

Call Syntax

Call subroutines with parentheses but without a leading &.

It's possible to call a subroutine without parentheses, if it has already been declared in the current namespace:

sub coerce;

# and later... my $expected_count = coerce $input, $INTEGER, $ROUND_ZERO; ...

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