You are trying to use symbolic refs. This is usually a bad idea (and you should always use strict to avoid it...), but in case you know what you're doing and insist:
the expression:
test-$number-$string Is like  'test' - $string - $number
So, in all of the three push statements, you're pushing to the array @{-9} (strings are like 0 in numeral context, both 'vol1' and 'vol2', and the - operator implies numberal context).
What you'd probably want to do is use them as strings, such as:
@{"test-$contextID-$vol[0]"}