If you need only part of the RE to work case-insensitively, look at the \l and \u metacharacters (for one char at a time) or the \L, \U, and \E metacharacters (for altering the case of chunks of characters). These are also described in detail in perlretut.

I don't think those help in making portions of RE's case insensitive, they will change portions of strings to upper or lower case. For case insensitive portions of an RE, use /some(?i:InSensiTIve)portion/