Something like this, you mean?

use strict;
use warnings;

my $offset = 0;     # Where are we in the string?

my $string = do {   # Grab the string.
    local $/;
    <DATA>;
};
my $numResults = 0;

while (1) {
    my $idxSummary = index($string, "SUMMARY", $offset);
    my $result = "";
    if ($idxSummary > -1) {
        $offset = $idxSummary + length("SUMMARY");
        my $idxDescription = index($string, "DESCRIPTION", $offset);
        if ($idxDescription == -1) {
            print "(Data malformed: missing DESCRIPTION line.)\n";
            last;
        }
        my $length = $idxDescription - $offset;
        $result = substr($string, $offset, $length);
        $offset = $idxDescription + length("DESCRIPTION");
        $result =~ s/^\s+|\s+$//g ; # Strip leading and trailing white
+space,
                                    # includng newlines.
        $numResults++;
    } else {
        print "(All done. $numResults result(s) found.)\n";
        last;
    }
    print " <$result>\n";
}

__DATA__
This is bogus data

SUMMARY
Event 1
DESCRIPTION
Lorem ipsum etc etc
This is bogus data

SUMMARY
Event 2
DESCRIPTION
Lorem ipsum
This is bogus data

SUMMARY
The Third Event
DESCRIPTION
Lorem ipsum
This is bogus data

SUMMARY
Event Number Four
DESCRIPTION
Lorem ipsum
[download]

It gives this output:

 <Event 1>
 <Event 2>
 <The Third Event>
 <Event Number Four>
(All done. 4 result(s) found.)
[download]

Update: Alternatively, if it's not really the indexes you care about, but only capturing those titles, how about this?

my @results = $string =~ m/SUMMARY\s*(.+?)\s*DESCRIPTION/g;
print map { " <$_>\n"} @results;
[download]