You're assigning just a reference (to the thawed content) to $hash, which goes out of scope at the end of the routine. In other words, the reference changes, but what it originally pointed to isn't changed.  Try

    %$hash = %{ thaw($hash2) };
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What you have is kind of like

#!/usr/bin/perl -l

my $href1 = { foo => "OK" };
my $href2 = $href1;         # $href1 and $href2 point to the same hash

print $href2->{foo};

$href2->{foo} = "NOT OK";   # modify entry

print $href2->{foo};

$href2 = { foo => "OK2" };  # $href2 now points to another new hash

print $href2->{foo};    
print $href1->{foo};        # $href1 still points to the old hash

__END__
OK
NOT OK
OK2
NOT OK
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