non-grapheme delimiters not properly prohibited

This is a bug report for perl from zefram@fysh.org,
generated with the help of perlbug 1.43 running under perl 5.37.9.


-----------------------------------------------------------------
<!--[Please describe your issue here]-->

perldiag\(1\) and perldeprecation\(1\) say that quotelike constructs no
longer \(since Perl 5\.30\) permit the use of a delimiter that doesn\'t
constitute a complete grapheme\. This prohibition doesn\'t entirely
work\. Suppose I use a vertical bar as a delimiter\, with the character
immediately following the opening delimiter being a combining
circumflex\, such that the two characters combine into a single
grapheme\:

```
$ perl -lwe $'use utf8; print length(q|\xcc\x82foo|)'
4
```

Perl is aware that there isn\'t a grapheme cluster boundary there\:

```
$ perl -lwe $'use utf8; print "|\xcc\x82" =~ /.\\b{gcb}./ ? "yes" : "no"'
no
```

<!--[Please do not change anything below this line]-->
<!------------------------------------------------------------------- -->


---
**Flags**
- category=core
- severity=low
---
**Perl configuration**
```
Site configuration information for perl 5.37.9:

Configured by zefram at Wed Mar  1 02:31:43 GMT 2023.

Summary of my perl5 (revision 5 version 37 subversion 9) configuration:
   
  Platform:
    osname=linux
    osvers=5.10.0-21-amd64
    archname=x86_64-linux-thread-multi
    uname='linux barba.rous.org 5.10.0-21-amd64 #1 smp debian 5.10.162-1 (2023-01-21) x86_64 gnulinux '
    config_args='-des -Dprefix=/home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52 -Duselargefiles -Dusethreads -Uafs -Ud_csh -Uusesfio -Uusenm -Duseshrplib -Dusedevel -Uversiononly -Ui_db'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
  Compiler:
    cc='cc'
    ccflags ='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2'
    optimize='-O2'
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
    ccversion=''
    gccversion='10.2.1 20210110'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=16
    longdblkind=3
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='cc'
    ldflags =' -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib
    libs=-lpthread -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -ldl -lm -lcrypt -lutil -lc
    libc=libc-2.31.so
    so=so
    useshrplib=true
    libperl=libperl.so
    gnulibc_version='2.31'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E -Wl,-rpath,/home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52/lib/5.37.9/x86_64-linux-thread-multi/CORE'
    cccdlflags='-fPIC'
    lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector-strong'


---
@INC for perl 5.37.9:
    /home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52/lib/site_perl/5.37.9/x86_64-linux-thread-multi
    /home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52/lib/site_perl/5.37.9
    /home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52/lib/5.37.9/x86_64-linux-thread-multi
    /home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52/lib/5.37.9

---
Environment for perl 5.37.9:
    HOME=/home/zefram
    LANG (unset)
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/zefram/usr/perl/perl_install/perl-5.37.9-i64-f52/bin:/home/zefram/usr/perl/util:/home/zefram/pub/x86_64-unknown-linux-gnu/bin:/home/zefram/pub/common/bin:/usr/bin:/bin:/usr/local/bin:/usr/games
    PERL_BADLANG (unset)
    SHELL=/usr/bin/zsh
```

• non-grapheme delimiters not properly prohibited by zefram via perl5-porters