Your code as written won't work, for at least two reasons. One, unless you've defined one, there is no SOME_DRIVE environment variable. Two, you'll want to put double-quotes around that last bit:

my $path = "$DRIVE_LETTER/tmp/$some_file";
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Depending on what your scripts do, though, you may not need to worry about what your current path is. And if you use relative paths, you shouldn't need to worry about drive letters whatsoever on Win2K.