Since posting this question, I came up with the following hack as a solution.
Any comments?

use Env;
sub GetPathVarSeparator{
    my( $len, $separator );

    #get the length of the first item in the path
    $len = length( (File::Spec->path)[0] );
    $separator = substr( $PATH, $len, 1 );
    return $separator;
}
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The idea is to use the difference between what File::Spec->path returns and the actual value of the $PATH variable. This correctly gives me ";" on win2k and ":" on my linux bash shell.