Understanding Subroutines - Perl

Now that we know what subroutines are, it’s time to look at how to define them and how to use them. First, let’s see how to create subroutines.

Defining a Subroutine

We can give Perl some code, and we can give it a name, and that’s our subroutine. Here’s how we do it:

sub example_subroutine {
   ...
}

There are three sections to this declaration:

1. The keywordsub. 
    
2. The name we’re going to give it. The rules for naming a subroutine are exactly those for naming variables; names must begin with an alphabetic character or an underscore, to be followed by zero or more alphanumerics or underscores. Uppercase letters are allowed, but we tend to reserve all-uppercase names for special subroutines. And again, as for variables, you can have a scalar$fred, an array@fred, a hash
   %fred, and a subroutinefred(), and they’ll all be distinct. 
    
3. A block of code delimited by curly braces, just as we saw when we were usingwhileandif. Notice that we don’t need a semicolon after the closing curly brace.

After we’ve done that, we can use our subroutine.

Before we go any further though, it’s worth taking a quick time-out to ponder how we name our subroutines. You can convey a lot about a subroutine’s purpose with its name, much like that of a variable. Here are some guidelines—not hard-and-fast rules—about how you should name subroutines:

1. If they’re primarily about doing something, name them with a verb—for example,summarize()ordownload(). 
    
2. If they’re primarily about returning something, name them after what they return—for example,greeting()orheader(). 
    
3. If they’re about testing whether something is true or not, give them a name that makes sense in anifstatement; starting withis_...orcan_...helps, or if that isn’t appropriate, name them with an adjective: for example,is_available(),valid(), orreadable(). 
    
4. Finally, if you’re converting between one thing and another, try and convey both things— traditionally this is done with a2or_to_in the middle:text2html(),meters_to_feet(). That way you can tell easily what’s being expected and what’s being produced.