unpack accepts as its first argument a template.
See perldoc -f pack on how to construct the template.

For this example, "a3" implies 3 ASCII characters. length($seq)/3 will return you how many such 3 character sets are there.

Hence, this template is equivalent to "a3a3a3a3" as long as it is needed. This splits $seq into elements each 3 Ascii character wide and returns it as a list.

Manav