Because you cannot pass arrays to subroutines; you can only pass a (single) list.

This reality stems from this:

( @a, @b ) = (1..10, 'a'..'b');

print @a;;
1 2 3 4 5 6 7 8 9 10 a b

print @b;;
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There is nothing to delimit the two different lists on the right hand side; so everything -- the numbers and the letters -- gets assigned to @a; nothing to @b;

Similarly, when you do this:

someFunction( @a, @b, $c );
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All the items from both arrays and the single scalar get concatenated into a single list, which inside the function you address as the array @_.

When you assign @_ to two arrays and a scalar inside your function:

    my ( @groupA, @groupB, $dataRef ) = @_;
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The information about which elements of @_ came from which argument has been lost; so everything gets assigned to @groupA and nothing to the other two variables.

Hence if you want to pass multiple arrays (or hashes; or combination thereof) to a function, you have to pass references to them and dereference internally.

It might sound like a limitation, but once you start using your function to sort arrays of any size, you'll be glad of the efficiency that results.