Thanks to Robert Lilja for the diagnosis; I had the same problem. Here is a
more generic patch:
--- /usr/lib/xymon/client/ext/kern~ 2023-03-01 03:21:11.000000000 +0100
+++ /usr/lib/xymon/client/ext/kern 2026-11-08 11:27:04.612321021 +0100
@@ -48,6 +48,7 @@
if ($newest_kernel_image) {
$newest_kernel_image =~ /vmlinu[xz]-(.*)$/;
my $kernel_image_release = $1;
+ $kernel_image_release =~ s/([.+])/\\$1/g;
my $kernel_image_read_command = "strings '$newest_kernel_image'";
if (-x '/usr/bin/dpkg' and `dpkg --print-architecture` =~ /sparc/) {
