Changing just a couple of small things to haukex's solution above should give you what you need:

use warnings;
use strict;
use File::ReadBackwards;

my $bw = File::ReadBackwards->new('/var/log/maillog') or die $!;
while (defined( my $line = $bw->readline )) {
    print $line if $line =~ /Passed CLEAN/;
}
$bw->close;
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