This turned out to be two bugs mangled into one.
First, _parse_varvals can deal with (var, undef), but not with (undef).
This is a problem because two of the trhee spots that call
_parse_varvals can return undef because of this:
map { $_ =~ m/^([^=]+)(?:=(.*))?$/ }
grep { $_ !~ m/^\s*#/ } # no # comment lines
split("\n", $mysqld_section)
The problem is twofold. First, we are not skipping empty or
whitespace-only lines. That means that the map will fail,
and pass an undef to _parse_varvals. So this ended up in
a triple fix: Make _parse_varvals deal with a sole undef,
skip empty/whitespace lines, and change that map to
map { $_ =~ m/^([^=]+)(?:=(.*))?$/ ? ($1, $2) : () }
so even if the regex fails in the future, no sole undef
will be passed down the chain.