Laravel Blade: concatenating variables as string inside old() function

I'm trying to implement the old() on a bunch of checkboxes generated by 2 foreach (one with key $day_key and one with $period_key)

<input type="checkbox" 
  name="availability[][]" 
   />

I need to print this string inside the old() function:

'availability["'.$day_key.'"]["'.$period_key.'"]'

but it doesn't work. How can I do it?

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