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Mastering Modern Perl: Writing Efficient and Maintainable Code

Iteration: Aliasing and Scoping

Explore how Perl's for loop creates aliases for iterator variables allowing in-place modification. Understand the risks of modifying read-only values and how iteration variable scoping, especially with $_, impacts code behavior and debugging. Gain insight into best practices for safe, idiomatic looping in Perl.

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Iteration and aliasing

The for loop aliases the iterator variable to the values in the iteration, such that any modifications to the value of the iterator modify the value in place:

Perl
use Test::More;
my @nums = 1 .. 10;
$_ **= 2 for @nums;
is $nums[0], 1, '1 * 1 is 1';
is $nums[1], 4, '2 * 2 is 4';
#...
is $nums[9], 100, '10 * 10 is 100';
done_testing()

This aliasing also works with the block-style for loop:

Perl
use Test::More;
my @nums = 1 .. 10;
for my $num (@nums) {
    $num **= 2;
}
is $nums[0], 1, '1 * 1 is 1';
is $nums[1], 4, '2 * 2 is 4';
# ...
is $nums[9], 100, '10 * 10 is 100';
done_testing()

It also works for ...