But getting <undef> as value

Is your Perl 5.12 or earlier, that is, more than six years old? There apparently was a bug with (?| ) that was fixed in v5.14 (this appears to be the commit). You should consider upgrading.

i am unable to understand regex!

Are you familiar with concepts such as non-capturing groups (?: ) and other basics like \s* etc.? If not, you probably want to read perlretut first. In fact, as far as I can tell even the advanced regex features I used are explained there (with more details on each in perlre):

• (?| ) - Alternative capture group numbering
• (?<name> ) - Named backreferences (see also %+)
• (?<= ) and (?! ) - Looking ahead and looking behind