Running this SSCCE I got the same error but removing the -UseApacheInclude=>1 fixed it. No idea why as docs suggest it should accept either format. HTH.

#!perl
use strict;
use Config::General;

printf "Module Version %s\n",$Config::General::VERSION;

my $conf = new Config::General(-ConfigFile => \*DATA,
  -InterPolateVars  => 1,
  #-UseApacheInclude => 1,
  -IncludeRelative  => 1
);

print $conf->save_string();
#news.conf
#<news>
#Some text
#</news>
__DATA__
<rightbar>
  baseurl   = baseurl
  <status>
     includefile = $baseurl/incl/LeftBar/status.html
  </status>
  <<include news.conf>>
</rightbar>
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