This is what I came up with.

while ( $text =~ m{
                   (?:[ ]{2}|\n|\A)  # Two spaces, newline, or string 
+start
                   (                 # start of phrase
                    (?:\S+\s)*       # nonspaces + ONE space, repeated
                    NOTICE           # literal NOTICE
                    (?:\s\S+)*       # one space + nonspaces, repeated
                   )                 # end of phrase
                   (?:[ ]{2}|\n|\z)  # two spaces, newline, or string 
+end
               }xmsg ) {

    print $1, "\n";
}
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