Win32's GetFullPathName and GetLongPathName will do that.
The former also converts the path to an absolute path.
The latter also expands short file names into long file names.

use Win32 qw( );

# Win32 doesn't export these, so we'll import them manually.
BEGIN {
   no strict 'refs';
   *$_ = \&{"Win32::$_"}
      for qw( GetLongPathName GetFullPathName );
}

my $path = "\\DOCUMENTS AND SETTINGS";

# If you wish to keep relative paths relative:
print GetLongPathName($path); 

# If you wish to make relative paths absolute:
print GetLongPathName(scalar(GetFullPathName($path)));
[download]

Note that Win32's GetCwd returns the path as provided to chdir.

Tested.