What do you expect the result will be for this?

print "test_array:  @$test_array " . #{${@{$test_array}}} . "\n";
print "test_array2: @$test_array2\n";
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Can you spot the error? Your code is equivalent to this (because of the comment # on the first line) -

print("test_array:  @$test_array " . print "test_array2: @$test_array2
+\n");
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Where your second print is evaluated before the first print.

If you want to print the index of the last element in the test array, use this ---> $#{@$test_array} instead