If I take away the complexity caused by returning from sub, then in both cases, 'x' got set:

use strict;
use warnings;
use Tie::Hash;

our %nottied = ('a'..'d');
tie our %tied, "Tie::StdHash";
%tied = ('a'..'d');
$_ = 'x' for (values %tied);
$_ = 'x' for (values %nottied);

print "tied is now: ", %tied, "\n";
print "nottied is now: ", %nottied, "\n";
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This gives you:

tied is now: cxax
nottied is now: cxax
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