Programming Perl, 4th Edition

Verbs

As is typical of your typical imperative computer language, many of the verbs in Perl are commands: they tell the Perl interpreter to do something. On the other hand, as is typical of a natural language, the meanings of Perl verbs tend to mush off in various directions depending on the context. A statement starting with a verb is generally purely imperative and evaluated entirely for its side effects. (We sometimes call these verbs procedures, especially when they’re user-defined.) A frequently seen built-in command (in fact, you’ve seen it already) is the say command:

say "Adam's wife is $wife{'Adam'}.";

This has the side effect of producing the desired output:

Adam's wife is Eve.

But there are other “moods” besides the imperative mood. Some verbs are for asking questions and are useful in conditionals such as if statements. Other verbs translate their input parameters into return values, just as a recipe tells you how to turn raw ingredients into something (hopefully) edible. We tend to call these verbs functions, in deference to generations of mathematicians who don’t know what the word “functional” means in normal English.

An example of a built-in function would be the exponential function:

my $e = exp(1);   # 2.718281828459 or thereabouts

But Perl doesn’t make a hard distinction between procedures and functions. You’ll find the terms used interchangeably. Verbs are also sometimes called operators (when built-in), or subroutines (when user-defined).^[17] But call them whatever you ...