This might be too late and not address your problem, but I have had similar troubles moving from Word 2003 to 2010, in particular your code example flunks for me too. After some anguish I recorded a macro to open a file with Word 2010 - and there was one big difference from 2003. Seems Word 2010 wants to see just a file name proper in the Open(), with a change of working directory beforehand. Doing that works for me. Here's a crude example you might try, note I've assumed a full path for $oldfile with forward slashes only:

use Win32::OLE; $Win32::OLE::Warn = 3;  
eval {$word = Win32::OLE->GetActiveObject('Word.Application')}; 
die "Word not installed" if $@; 
unless (defined $word) { $word = Win32::OLE->new('Word.Application', 
sub {$_[0]->Quit;}) or die "Oops, cannot start Word"; } 
$word->Activate; 
$word->{visible} = 1;
$oldfile =~ m!^(.+?)/([^/]+)$!;
my $dir = $1 . '/';
my $name = $2;
$word->ChangeFileOpenDirectory($dir);
my $doc = $word->Documents->Open("$name");
...
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