You have an error with precedence in your regular expression. The alternation (|) doesn't know where you want it to stop. See YAPE::Regex::Explain if you want a module that explains regular expressions to you.

Your regular expression currently matches like

(?-imsx:^REGISTER|INVITE|BYE\s\w\.\w\sSIP/2.0)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  REGISTER                 'REGISTER'
----------------------------------------------------------------------
 |                        OR
----------------------------------------------------------------------
  INVITE                   'INVITE'
----------------------------------------------------------------------
 |                        OR
----------------------------------------------------------------------
  BYE                      'BYE'
----------------------------------------------------------------------
  \s                       whitespace (\n, \r, \t, \f, and " ")
----------------------------------------------------------------------
  \w                       word characters (a-z, A-Z, 0-9, _)
----------------------------------------------------------------------
  \.                       '.'
----------------------------------------------------------------------
  \w                       word characters (a-z, A-Z, 0-9, _)
----------------------------------------------------------------------
  \s                       whitespace (\n, \r, \t, \f, and " ")
----------------------------------------------------------------------
  SIP/2                    'SIP/2'
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  0                        '0'
----------------------------------------------------------------------
[download]

Maybe you want the Net::SIP module instead of writing your own module?

As an aside, putting your code between <code>...</code> tags makes the code render and download nicely for those who want to try it.

Update: YAPE::Regex::Explain installed and I updated the explanation