while(<DATA>) {
    if (m/<ORIG DATA ABOVE MUNGE BELOW>/) {
        $munge .= $_ while <DATA>;  # slurp
    } else {
        $orig .= $_;
    }
}

##</code><code>##

my ($orig,$munge) = load_file($file);  # munge may be NULL
my $data          = parse($orig);      # process current format data only
my $cur_format    = serialise($data);  # output current format
write_file($orig, $current_format);    # write to file with separator
my $invalid       = eq_or_diff($munge,$cur_format); 
print "$file\n$invalid\n" if $invalid; # diff output, null if OK