It's too late at runtime, the rest of the script already got compiled. So "Object" is taken as a bareword string literal, and nothing can change that.

As a constant is nothing but a sub, you can use a similar sub that just returns the value of a scalar. You can always change the value for the scalar.

my $class = "Gemini::Model::Object";
sub Object () {
    return $class;
}
print ref Object->new();
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You can always still change the value of that scalar.

But why the obfuscation? If you want to use a variable class, just say so. Perl lets you do that.

my $class = "Gemini::Model::Object";

print ref $class->new();
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Looks fine to me.