There are two approaches to writing portable code to deal with paths. All systems running perl can cope with POSIX filenames, i.e. using '/' as a path separator. This works except when performing textual manipulation on a file spec in native format, though Windows can sometimes cope with a mixture of forward slashes and backslashes.

The other approach is to use File::Spec, Cwd and friends in conjunction with the native syntax. If you do this, stick to native syntax throughout. Note that some OSes e.g. VMS, don't have a single 'path separator' but something more complicated.

For more on this, see perldoc perlport and my YAPC portability talk slides.

By the way, I don't think your approach will work in the OP's case.__FILE__ is an alias for $0 as far as I am aware, so this may not contain any path separators at all.