Please go to the professor and tell them that they gave you incorrect specifications. They define even elements as

they have subscript values of 2, 4, 6, 8, 10

but it's not correct! 20 is an even integer, for example, but does not have any of the subscripts in the specs. Maybe you'll want to use this definition:

they have subscript values of 2, 4, 6, 8, 0

or better test for oddity using the remainder of an integer division by 2 (see the % operator in perlop).

I'd also suggest the professor to add "20" to the list of numbers - when you find a bug, add a test to the test suite.