use strict;
use warnings;

my $last_seen;

for my $line ( <DATA> ) {
    $last_seen = $line if $line =~ /YYY/;
    print $last_seen if $line =~ /ABC/;
}

__DATA__
something
something
YYY first
something
something
YYY second
something
ABC
something

##</code><code>##

$ perl ~/monks/1223838.pl
YYY second