my $dbh = "123456"; # just as a test, instead of DBI->connect(....);

if ($dbh != valid_DBI_handle()) {
  print "please supply a PROPER reference to a DBI object, or whatever it's called\n";
} else {
  # okay, dbh actually points to a real db connection
  print "dbh is truly valid";
}

##</code><code>##

if (!$dbh) {
  die "no db connection!";
} else {
  $dbh->run_some_command();
}