use strict;
if ($^O ne 'MSWin32') {  # NOT on windows?
  require POSIX;
  require Filesys::SmbClient;
}

...

sub OpenFile {
  my ($server, $path, $username .... ) = @_;
  if ($^O eq 'MSWin32') {
    # open file directly as //server/path on Windows
    ...
  } else {
    # open file via smbclient on *nix
    ...
  }
}