You wrote:

my $x = 123;

( async {
   $x = 456;
   say $X;  # 456
} )->join();

say $x;  # 123
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but I don't think you tried it; on the line you annotate with # 456, you probably mean to say $x, but you put say $X. Tragically, Perl is case-sensitive when it comes to variables.