Well spotted, you're not wrong.

That comes about because this is an as yet incomplete modification of an old subroutine with different goals. The bugs I encountered meant that I haven't gotten around to trying finishing the modifications.

But that has nothing to do with the error, as they are reproduced by this simplification:

#! perl -slw
use strict;
use threads;
use threads::shared;

sub test {
    my @results :shared;
    async {
        @results = 1 .. 10;
    }->join;
    
    print "@results";
    return @results;
}

print test();

__END__
C:\test>junk50
1 2 3 4 5 6 7 8 9 10
Use of uninitialized value in print at C:\test\junk50.pl line 16.
Use of uninitialized value in print at C:\test\junk50.pl line 16.
Use of uninitialized value in print at C:\test\junk50.pl line 16.
Use of uninitialized value in print at C:\test\junk50.pl line 16.
Use of uninitialized value in print at C:\test\junk50.pl line 16.
Use of uninitialized value in print at C:\test\junk50.pl line 16.
Use of uninitialized value in print at C:\test\junk50.pl line 16.
Use of uninitialized value in print at C:\test\junk50.pl line 16.
Use of uninitialized value in print at C:\test\junk50.pl line 16.
Use of uninitialized value in print at C:\test\junk50.pl line 16.
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I didn't switch to this as my example because I could not reproduce the circumstances of the "modification of a readonly value" with this.