Use the s and/or m operator, documented in perlre:

• m

  Treat string as multiple lines. That is, change "^" and "$" from matching the start or end of the string to matching the start or end of any line anywhere within the string.

• s

  Treat string as single line. That is, change "." to match any character whatsoever, even a newline, which normally it would not match.

  Used together, as /ms, they let the "." match any character whatsoever, while still allowing "^" and "$" to match, respectively, just after and just before newlines within the string.