The problem is with using the expression (%x = reverse %h) in list context.

Notice that when I try to use it to make an anonymous array:

use Data::Dumper;
my %h = (1 => 2, a => "b", c => 2);

print Dumper [ my %x = reverse %h ];
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I get a Bizarre copy of ARRAY in anonlist (perl v5.8.3). Same if I try to do

print Dumper { my %x = reverse %h };
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I have no idea what (my %x = reverse %h) evaluates to in list context, but whatever it is, Perl doesn't like it. The fact it doesn't die with the Bizarre copy error when using that expression as function arguments might be a bug...

Anyway, back to your original question, if you split the two reverses into two statements like

%h = reverse do { my %q = reverse %h; %q };
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you'll get what you expect.