... what (?i) is used for ...

This regex operator turns on case insensitivity until the end of the current matching scope. See  (?adlupimsx-imsx) and  (?^alupimsx) in Extended Patterns. It has the same effect (but with scope control) as the  /i global pattern modifier; see Modifiers. (Both in perlre.) Aside from its scope flexibility, I prefer to use  (?i) rather than the  /i modifier because of its greater general readability.