use strict;
use warnings;

my $events = <<'END_EVENTS';
d1:v1
d2:v2
d4:v4
d1:v1
d2:v2
d4:v4
END_EVENTS

open my $eventsIn, '<', \$events or die "Failed to open event file: $!";

my $d2_value;
my $d1_value;
my @eventList;

while (<$eventsIn>) {
    chomp;

    my $title;

    if (m/d1/i) {
        ($title, $d1_value) = split /:/;

    } elsif (m/d2/i) {
        ($title, $d2_value) = split /:/;

    } elsif (m/d4/i) {
        push @eventList, "$d1_value$d2_value\n";
    }
}

close $eventsIn;

print @eventList;

##</code><code>##

v1v2
v1v2