Munin/Master/Utils.pm calls (line 689):
            unless Munin::Common::Config::cl_is_keyword($tmpvar);

This is a legacy call, have been removed in the .999 current, and it seems that
it's been removed from Config.pm but not from Utils.pm (and Utils.pm was completely
reworked since).

Until then, either rewrite Utils.pm relevant line (I didn't have the time to
reverse-engineer what they want now), or
--- /usr/share/perl5/Munin/Common/Config.pm~    2022-02-10 01:24:56.000000000 +0100
+++ /usr/share/perl5/Munin/Common/Config.pm     2023-11-22 10:25:04.877859590 +0100
@@ -150,6 +150,14 @@

 my %bools = map { $_ => 1} qw(yes no true false on off 1 0);

+sub cl_is_keyword {
+    # Class-less version of is_keyword for legacy code.
+    my ($word) = @_;
+
+    return defined $legal{$word};
+}
+
+
 sub is_keyword {
     my ($self, $word) = @_;