You need the /s modifier so that . matches newlines. See perldoc perlre. But it won't match anyway, because there isn't a %> sequence before the first single quote. This version optionally matches the start of the string instead of %> and the end instead of <%:

s/
  (
    (?:        # non capturing parens
      ^|%>  # start of string or %>
    ) 
    .* 
  )
  '
  (.*
     (?:<%|$) 
  )
/$1 SOMESTRING $2/gsx # add s and x modifiers
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I'm not at all sure this is a good or robust solution however, and I'm looking forward to the other replies.