In expanded commented /x form:

  $textfield =~ s/         # substitute
                \015\012   # a CRLF sequence (DOS, MIME...)
                |          # or
                \015       # a lone LF (mac)
                |          # or   
                \012       # a lone LF (unix)
                /          # with literal ''
                /xg;       # /x allow comments, /g do globally