It could be as simple as this:

while( $string =~ m/(SUMMARY.+?DESCRIPTION)/ig ) {
    print "Summary: $1\n"
}
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The while() loop continues looping as long as its condition is true. In this case, the condition is whether or not a match occurs. And by using the /g modifier, multiple matches can be detected within the same string. Each time a match occurs the internal position marker advances so that the next match will come after the current match. perlre and perlretut describe the use of the /g modifier in greater detail.

I removed the leading and trailing instances of .+?, because I don't think they were actually doing anything useful for you, and they potentially get in the way of repeated matches (the /g modifier). As you're probably aware, .+? says to non-greedily match one or more characters. If your only reason for that is to assure that there is at least one character separating matches, you could just do it with a single dot to the left and right of the capture. We're not trying to anchor to anything outside of the capture anyway, from what I can tell.