$_ read-only issue on "21". Not new, always has been

[email protected] ("H.Merijn Brand" via perl5-porters) Thu, 16 Apr 2026 09:30:04 +0200
Newsgroups	perl.perl5.porters
Message-ID	<20260416093004.1f247598@pc09>
I am really stunned that I have never hit this before, as it exists for
all versions of perl, but for some with a twist ...


--8<--- test.pl
#!/usr/bin/perl

use strict;
use warnings;

our $VERSION = "1.00";

sub foo {
    my $x = shift;
    my $n = 0;
    my $f = $0;
    open my $fh, "<", $f or return;
    while (<$fh>) {
	$n++;
	}
    close $fh;
    } # foo

foo ($_) for "00".."13","21",30..50;
-->8---

🐧 perl test.pl
Modification of a read-only value attempted at test.pl line 13.

Adding `local $_;` before the `while (<$fh>) {` is a workaround
but somehow my brain always thought that $_ would have been localized
or lexicalized inside the while. Obviously not

Weird is that - at least on my side - this read-only fail *ONLY*
happens on "21" If I leave out "21", all PASSs on ALL perl releases

Now check when that was introduced ...

🐧 perl-all test.pl
Running perl-all test.pl
On 621 versions of perl
 ranging from perl5.6.0 to tperl5.43.7
=== base/perl5.6.0      5.006    x86_64-linux
Modification of a read-only value attempted at test.pl line 13.
Exit status: 65280
=== base/perl5.6.1      5.006001 x86_64-linux-perlio
Modification of a read-only value attempted at test.pl line 13.
Exit status: 65280
=== base/tperl5.6.1     5.006001 x86_64-linux-thread-multi-ld-perlio
Modification of a read-only value attempted at test.pl line 13.
Exit status: 65280
:
:
=== base/tperl5.28.1    5.028001 x86_64-linux-thread-multi-ld
Modification of a read-only value attempted at test.pl line 13.
Exit status: 6400
=== base/cperl5.28.2    5.028002 x86_64-linux-thread-multi-ld
Modification of a read-only value attempted PV("21"\0) at sv.c:5284 at test.pl line 13.
Exit status: 65280
:
:
=== base/tperl5.28.3    5.028003 x86_64-linux-thread-multi-ld
Modification of a read-only value attempted at test.pl line 13.
Exit status: 6400
=== base/cperl5.28      5.028002 x86_64-linux-thread-multi-ld
Modification of a read-only value attempted PV("21"\0) at sv.c:5284 at test.pl line 13.
Exit status: 65280
:
:
=== base/tperl5.29.0    5.029000 x86_64-linux-thread-multi-ld
Modification of a read-only value attempted at test.pl line 13.
Exit status: 6400
cperl5.29.1: /lib64/libffi.so.7: version `LIBFFI_BASE_7.0' not found (required by cperl5.29.1)
=== base/cperl5.29.1
cperl5.29.1: /lib64/libffi.so.7: version `LIBFFI_BASE_7.0' not found (required by cperl5.29.1)
Exit status: 256
:
:
=== base/tperl5.29.10   5.029010 x86_64-linux-thread-multi-ld
Modification of a read-only value attempted at test.pl line 13.
Exit status: 6400
cperl5.29: /lib64/libffi.so.7: version `LIBFFI_BASE_7.0' not found (required by cperl5.29)
=== base/cperl5.29
cperl5.29: /lib64/libffi.so.7: version `LIBFFI_BASE_7.0' not found (required by cperl5.29)
Exit status: 256
=== base/tperl5.43      5.043007 x86_64-linux-thread-multi-ld
Modification of a read-only value attempted at test.pl line 13.
Exit status: 6400
=== /usr/bin/perl       5.042000 x86_64-linux-thread-multi
Modification of a read-only value attempted at test.pl line 13.
Exit status: 6400
=== /pro/bin/perl       5.042002 x86_64-linux-thread-multi-quadmath
Modification of a read-only value attempted at test.pl line 13.
Exit status: 6400

-- 
H.Merijn Brand  https://tux.nl   Perl Monger   http://amsterdam.pm.org/
using perl5.00307 .. 5.37        porting perl5 on HP-UX, AIX, and Linux
https://tux.nl/email.html http://qa.perl.org https://www.test-smoke.org
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