variable-use-via-copy.json

{
    "$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": { },
    "variables": {
        "comment": "this works with property copy as well", 
        "copy": [
            {
                "name": "arrayTest",
                "count": 5,
                "input": "[concat('item', copyIndex('arrayTest', 1))]"
            }
        ]
    },
    "resources": [],
    "outputs": {
        "foo": {
            "type": "array",
            "value": "[variables('arrayTest')]"
        }
    }
}