The package is provided in CentOS Stream 10.

perl-XString does not appear to be available in CentOS Stream 10 for aarch64:

https://koji.fedoraproject.org/koji/taskinfo?taskID=123056519 (perl-DateTime):

DEBUG util.py:461:  Error: 
DEBUG util.py:461:   Problem 1: conflicting requests
DEBUG util.py:461:    - nothing provides perl(XString) needed by perl-Specio-0.48-5.el10_0.noarch from build
DEBUG util.py:461:   Problem 2: package perl-DateTime-TimeZone-2.62-4.el10_0.noarch from build requires perl(Specio::Library::String), but none of the providers can be installed
DEBUG util.py:461:    - package perl-DateTime-TimeZone-2.62-4.el10_0.noarch from build requires perl(Specio::Library::Builtins), but none of the providers can be installed
DEBUG util.py:461:    - conflicting requests
DEBUG util.py:461:    - nothing provides perl(XString) needed by perl-Specio-0.48-5.el10_0.noarch from build
DEBUG util.py:461:   Problem 3: package perl-DateTime-Locale-1.43-1.el10_0.noarch from build requires perl(Specio::Library::String), but none of the providers can be installed
DEBUG util.py:461:    - package perl-DateTime-Locale-1.43-1.el10_0.noarch from build requires perl(Specio::Declare), but none of the providers can be installed
DEBUG util.py:461:    - conflicting requests
DEBUG util.py:461:    - nothing provides perl(XString) needed by perl-Specio-0.48-5.el10_0.noarch from build

I have raised https://issues.redhat.com/browse/CS-2515

perl-XString is now in CentOS Stream master mirror for all achitectures.
But it still missing from a mirror used by EPEL <https://kojipkgs.fedoraproject.org//work/tasks/2661/123292661/root.log>. It's in primary.xml, but the RPM file is not there yet,

I've been getting a different error:
https://kojipkgs.fedoraproject.org//work/tasks/2532/123292532/root.log

Same thing for over an hour now so probably not a mirror update in progress.

That's the same issue. repomd.xml and primary.xml are updated. Packages directory isn't.

Seems that it's fixed.

Yes, it is.