if ($^O eq 'MSWin32') {
  &Net_SMTP(...);
}
else {
  &TrySendmail(...);
}

sub TrySendmail {
  if (open (SENDMAIL, "| /usr/bin/sendmail ....")) {
    # versende mail mit sendmail
  }
  else {
    # sonst irgendwas
  }
}