use Getopt::Long;
my $configfile;
my @ARGVcopy = @ARGV; # because 1st pass eats all options in @ARGV!!
{
  local $SIG{__WARN__} = sub { }; # don't complain when see options relevant to the 2nd pass
  Getopt::Long::GetOptions('--config=s', \$configfile); # no check! it will fail
}
die "config needed" unless $configfile;
my $confighash = parse_configfile($configfile);
# 2nd pass with restored ARGV
@ARGV = @ARGVcopy;
Getopt::Long::GetOptions(
 'config=s' => \my $dummy, #<<<< this belongs to 1st phase but is still in cmdline (@ARGV)
 '--max-value=i' => \$confighash->{'a'}->{'b'}->{'max-value'},
) or die "error cmd line params";