Background

    sub routine {
        my $longstring = \shift;
        print $$longstring;
    }

is more readable than

    sub routine {
        print $_[0];    # longstring
    }

Unfortunately, this added readability brings with it new and exciting issues, detailed in the next section.

Problems with "\shift"

Memory leak since Perl 5.22

Issue #126676 was introduced in Perl 5.21.4 and is triggered when "\shift" is used. The bug has not been resolved as of Perl 5.28.

In short, the bug causes the ref counter for the aliased variable to be incremented when running the subroutine, but it is not subsequently decremented after the subroutine returns. In addition to leaking memory, this issue can also delay the cleanup of objects until Global Destruction, which can cause further issues.

For more information, see <https://rt.perl.org/Public/Bug/Display.html?id=126676>.

Devel::Cover crashes

A separate, longstanding issue in Devel::Cover (since at least 1.21), causes test code to segfault occasionally. This prevents the coverage data from being written out, resulting in bad metrics.

The bug itself isn't actually caused by "\shift", instead it shows up in code like the following:

    sub myopen {
        open ${ \$_[0] }, ">test";
    }
    

However, this code would rarely be seen in production. It would more likely manifest with "\shift", as it does below:

    sub myopen {
        my $fh = \shift;
        open $$fh, ">test";
    }
    

So while "\shift" isn't the cause, it's often associated with the problem.

For more information, see <https://github.com/pjcj/Devel--Cover/issues/125>.