The Slice => {} option retrieves all column names for each row, as it returns hashes. You can then use these as the keys.

An (somewhat ugly) alternative is to add WHERE 1 = 0 to the query and run it and afterwards ask the statement handle for the names of its columns. This has the advantage of keeping all columns in the same order as the select clause, but has the disadvantage of needing to shim the clause into an existing string.

DBI has more on the topic.