Apparently, the patch you need to apply to fix this is 9cddf794fc5 (included in 5.10.1). (Update: Not really an answer to your "why" question, though, sorry.)

$ git checkout -f v5.10.0
$ git clean -dxf
$ patchperl
$ git show 9cddf794fc5 | git apply
$ sh Configure -des -Dusethreads
$ make
# etc.

[download]

I'm not an expert on the internals, so don't ask me why :-/

$ Porting/bisect.pl -Dusethreads --target=miniperl --expect-fail \
    --start=v5.10.0 --end=v5.26.0 -e 'eval q("x"=~?x?)'
...
9cddf794fc52f08a145668164b1e3c15c91a713f is the first bad commit
commit 9cddf794fc52f08a145668164b1e3c15c91a713f
Author: Nicholas Clark <nick@ccl4.org>
Date:   Sun Jan 6 13:35:38 2008 +0000

    Clarify the intent of the code in Perl_op_clear. Under ithreads, a
+void
    calling sv_ivset twice. As a side effect, eliminate PM_GETRE_SAFE
    and PM_SETRE_SAFE, as we're doing "safe" explicitly in Perl_op_cle
+ar().
    
    p4raw-id: //depot/perl@32867

:100644 100644 c07a1a790672e577a6fdbabb140ef2a93dd27038 569a15910691ee
+2da5a6e75e70bad3e73c274cd6 M    op.c
:100644 100644 14b8e6ef246b958dae153c034d93ca1319af415f 264dd52ab8ebf0
+184c1d5a13743aaf2d2e9ab62e M    op.h
bisect run success

$ git bisect reset
$ cpanm Devel::PatchPerl

$ git checkout -f v5.10.0
$ git clean -dxf
$ patchperl
$ sh Configure -des -Dusethreads
$ make miniperl
$ ./miniperl -e 'eval q( "x" =~ ?x? ) and print "OK!\n"'
Modification of a read-only value attempted at -e line 1.
$ git show 9cddf794fc5 | git apply
$ make miniperl
$ ./miniperl -e 'eval q( "x" =~ ?x? ) and print "OK!\n"'
OK!

$ git tag --contains 9cddf794fc5
...
perl-5.10.1
...

[download]

A few minor updates.

In reply to Re: Threaded 5.10.0 + ?PATTERN? = Modification of a read-only value attempted by haukex
in thread Threaded 5.10.0 + ?PATTERN? = Modification of a read-only value attempted by choroba

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