I think what you are trying to do is:

 $Row->{ Cells }     =[ @{ $Row->{ Cells } }[  @$FieldOrder  ] ];
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The @{ $Row->{ Cells } } is the array , which you slice using [ @$FieldOrder ];
Then generate an anonymous array ref (The outermost [ ] )
The anon array ref is assigned to the scalar $Row->{ Cells } ,