Is there a way to do this that is portable between Win32 and Linux

Yes of course. You can check the $^O variable to see whether you are on Windows or Linux platform, and act accordingly.

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
    ...
  }
}
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