my $volume_name = "/Volumes/Volume Name/";

for (glob "$volume_name/*") {
    if (-d) {
        say $_;
        find(\&wanted, $_);
    }
}
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You have a trailing slash on the assignment to $volume_name and then you add another slash when you use $volume_name with glob, ergo, two slashes.

You could try it like this:

find \&wanted, grep -d, glob '/Volumes/Volume Name/*';
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