It appears that perl does not see the use of $x in foo, and thus does not close foo on $x. But once you put in the "print $x" there, it does close foo on $x, and then the anonymous sub can close on the same $x.

One workaround would be:

use strict;
use warnings;

sub run_this { shift->() }

{
    my $x = "hello\n";
    
    sub foo {
        {
            no warnings;
            $x
        }
        run_this(
                 sub {
                     print $x;
                 });
    }
}

foo();
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Note the useless use of $x in a void context, which gets masked away with the no warnings bit. It does the same as yours as far as the problem is concerned (forcing perl to close foo on $x), but with no side effects going to standard-out ;-)