Manpages - if.3perl

The if module is used to conditionally load another module. The construct:

use if CONDITION, “MODULE”, ARGUMENTS;

… will load MODULE only if CONDITION evaluates to true; it has no effect if CONDITION evaluates to false. (The module name, assuming it contains at least one ::, must be quoted when use strict "subs"; is in effect.) If the CONDITION does evaluate to true, then the above line has the same effect as:

use MODULE ARGUMENTS;

For example, the Unicode::UCD module’s charinfo function will use two functions from Unicode::Normalize only if a certain condition is met:

use if defined &DynaLoader::boot_DynaLoader, “Unicode::Normalize” => qw(getCombinClass NFD);

Suppose you wanted ARGUMENTS to be an empty list, i.e., to have the effect of:

use MODULE ();

You can’t do this with the if pragma; however, you can achieve exactly this effect, at compile time, with:

BEGIN { require MODULE if CONDITION }

The no if construct is mainly used to deactivate categories of warnings when those categories would produce superfluous output under specified versions of perl.

For example, the redundant category of warnings was introduced in Perl-5.22. This warning flags certain instances of superfluous arguments to printf and sprintf. But if your code was running warnings-free on earlier versions of perl and you don’t care about redundant warnings in more recent versions, you can call:

use warnings; no if $] >= 5.022, q|warnings|, qw(redundant); my $test = { fmt => “%s”, args => [ qw( x y ) ] }; my $result = sprintf $test->{fmt}, @{$test->{args}};

The no if construct assumes that a module or pragma has correctly implemented an unimport() method Ω- but most modules and pragmata have not. That explains why the no if construct is of limited applicability.