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4.10. The use strict Pragma

Perl tends to be a pretty permissive language. But maybe you want Perl to impose a little discipline; that can be arranged with the use strict pragma.

A pragma is a hint to a compiler, telling it something about the code. In this case, the use strict pragma tells Perl's internal compiler that it should enforce some good programming rules for the rest of this block or source file.

Why would this be important? Well, imagine that you're composing your program, and you type a line like this one:

$bamm_bamm = 3;  # Perl creates that variable automatically

Now, you keep typing for a while. After that line has scrolled off the top of the screen, you type this line to increment the variable:

$bammbamm += 1;  # Oops!

Since Perl sees a new variable name (the underscore is significant in a variable name), it creates a new variable and increments that one. If you're lucky and smart, you've turned on warnings, and Perl can tell you that you used one or both of those global variable names only once in your program. But if you're merely smart, you used each name more than once, and Perl won't be able to warn you.

To tell Perl that you're ready to be more restrictive, put the use strict pragma at the top of your program (or in any block or file where you want to enforce these rules):

use strict;  # Enforce some good programming rules

Now, among other restrictions,[115] Perl will insist that you declare every new variable with my:[116]

[115]To learn about the other restrictions, see the documentation for strict. The documentation for any pragma is filed under that pragma's name, so the command perldoc strict(or your system's native documentation method) should find it for you. In brief, the other restrictions require that strings be quoted in most cases, and that references be true (hard) references. Neither of these restrictions should affect beginners in Perl.

[116]There are some other ways to declare variables, too.

my $bamm_bamm = 3;  # New lexical variable

Now if you try to spell it the other way, Perl can complain that you haven't declared any variable called $bammbamm, so your mistake is automatically caught at compile time.

$bammbamm += 1;  # No such variable: Compile time error

Of course, this applies only to new variables; Perl's builtin variables, such as $_ and @_ never need to be declared.[117]

[117]And, at least in some circumstances, $a and $b won't need to be declared, because they're used internally by sort. So if you're testing this feature, use other variable names than those two. The fact that use strict doesn't forbid these two is one of the most frequently reported non-bugs in Perl.

If you add use strict to an already-written program, you'll generally get a flood of warning messages, so it's better to use it from the start, when it's needed.

Most people recommend that programs that are longer than a screenful of text generally need use strict. And we agree.

From here on, most (but not all) of our examples will be written as if use strict is in effect, even where we don't show it. That is, we'll generally declare variables with my where it's appropriate. But, even though we don't always do so here, we encourage you to include use strict in your programs as often as possible.

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