Why does Perl::Critic dislike using shift to populate subroutine variables?

Question

Lately, I've decided to start using Perl::Critic more often on my code. After programming in Perl for close to 7 years now, I've been settled in with most of the Perl best practices for a long while, but I know that there is always room for improvement. One thing that has been bugging me though is the fact that Perl::Critic doesn't like the way I unpack @_ for subroutines. As an example:

sub my_way_to_unpack {
    my $variable1 = shift @_;
    my $variable2 = shift @_;

    my $result = $variable1 + $variable2;
    return $result;
}

This is how I've always done it, and, as its been discussed on both PerlMonks and Stack Overflow, its not necessarily evil either.

Changing the code snippet above to...

sub perl_critics_way_to_unpack {
    my ($variable1, $variable2) = @_;

    my $result = $variable1 + $variable2;
    return $result;
}

...works too, but I find it harder to read. I've also read Damian Conway's book Perl Best Practices and I don't really understand how my preferred approach to unpacking falls under his suggestion to avoid using @_ directly, as Perl::Critic implies. I've always been under the impression that Conway was talking about nastiness such as:

sub not_unpacking {
    my $result = $_[0] + $_[1];
    return $result;
}

The above example is bad and hard to read, and I would never ever consider writing that in a piece of production code.

So in short, why does Perl::Critic consider my preferred way bad? Am I really committing a heinous crime unpacking by using shift?

Would this be something that people other than myself think should be brought up with the Perl::Critic maintainers?

Answer 1

The simple answer is that Perl::Critic is not following PBP here. The book explicitly states that the shift idiom is not only acceptable, but is actually preferred in some cases.