But when part of main module, this does not work.

Maybe you should show that instead

Here below is the actual snippet from the main module. This does not work.

    {
        # Need to add tags of <vom> and <\vom> at the
        # start and end of the file
        # Doing this below, but in a different block
        # of code to not disturb ARGV
        $out->[1] =~ s/\\/\\\\/g; # for windows
        local @ARGV = ($out->[1]);
        local $^I = '.bk';
        while (<>) 
        {
            if ($. == 1)
            {
                print "<vom>\n";
            }
            else
            {
                print;
            }
        }
        open(APP, ">>$out->[1]");
        print APP "\n</vom>\n";
        close(APP);
    }
[download]

$out->[1] =~ s/\\/\\\\/g; # for windows
[download]

What is this line supposed to do? What does $out->[1] contain, and what should it contain afterwards?

I think you're making it way too complicated. In-place editing, when you don't know what you're doing, can wipe out your files. And besides, it's not really editing in-place. It makes an optional backup copy of the file, but it always writes a new file.

There's probably a module for this (such as Iterator::Diamond, noted here), but it's easy enough to do yourself also. Here's something simple, that is probably more reliable than what you're doing.

# Assumes you are using strict -- RIGHT?
{
    # Need to add tags of <vom> and <\vom> at the start and 
    # end of the file.

    # Changing '\' to '\\' is not wrong, but you can also use
    # '/' on Windows. Perl handles it just fine, and it's a lot
    # easier to read. It's optional here since you're not dealing
    # with the filenames directly.
    $out->[1] =~ s{\\}{/}g;

    # First, create a backup name and rename the original file. This
    # creates your backup file, and we'll create your "original" in
    # a moment.
    my $bk = $out->[1] . ".bk";
    rename $out->[1], $bk;

    # Open the backup for input.
    open my $f, '<', $bk or die $!; 

    my $data; # Use this to capture data.
    # Old-style slurp mode.
    {
        local $/ = undef;
        $data = <$f>;
    }

    # Now open the original filename, reusing the filehandle, and
    # write to it.
    open $f, '>', $out->[1] or die $!;
    print $f "<vom>\n", $data, "\n</vom>\n";
    close $f;                    # All done.
}
[download]

--marmot

Yeah, simply use Iterator::Files or Iterator::Diamond