Try adding a second file type specification and see if the error goes away on Win32.

Quite a while back (I'm unsure of which version of Tk was installed), I recall getting an error in a similar circumstance. I just found a comment in my old code that said that I needed to add a second file type for the code to run properly in Win32. I also seem to recall that the code in question ran fine on my Mac (and likely also ran fine on linux). So, try something like this:

my $Type_P4SM = [
        [ 'P4SM', '.p4sm' ],
        [ 'All Files', '*.*']
    ];
[download]

Please report back if this fixes your problem, as it may inspire me to file a bug report. I just took a quick look through the list of active bugs for Tk and did not see any bugs that looked similar to this one.