lib/PublicInbox/Smsg.pm | 2 +-

diff --git a/lib/PublicInbox/Smsg.pm b/lib/PublicInbox/Smsg.pm
index 2b72e8b5ae804e283cbd4711f2a001261da9355c..b4cc2ecb1afe6e4b2a224138cb39a6eff8bdca18 100644
--- a/lib/PublicInbox/Smsg.pm
+++ b/lib/PublicInbox/Smsg.pm
@@ -83,7 +83,7 @@ 		if (length($ref) > PublicInbox::MID::MAX_MID_SIZE) {
 			warn "References: <$ref> too long, ignoring\n";
 			next;
 		}
-		push(@keep, $ref) unless $seen{$ref}++;
+		$seen{$ref} //= push(@keep, $ref);
 	}
 	$smsg->{references} = '<'.join('> <', @keep).'>' if @keep;
 	\@keep;