Section 4.8. The use strict Pragma

4.8. The use strict Pragma

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

] Bet you hadnt noticed.

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 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 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,^[*] Perl will insist that you declare every new variable, usually done with my:^[]

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

     my $bamm_bamm = 3;  # New lexical variable

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 fatal error

Of course, this applies only to new variables; you don't need to declare Perl's built-in variables, such as $_ and @_.^[] If you add use strict to a previously written program, youll generally get a flood of warning messages, so it's better to use it from the start when it's needed.

^[] In some circumstances, you dont want to declare $a and $b 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.

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. Though we don't always do so here, we encourage you to include use strict in your programs as often as possible.