my $dbh = DBI->connect(
  "DBI:mysql:database=$db_name;host=$DB_HOST;mysql_client_found_rows=1",
  "$DB_USER", "$DB_PASS"
) or die $DBI::errstr;

my $rows = $dbh->do(qq~
  update mytable set myvalue = ? where mykey = ?
  ~, undef, $myvalue, $mykey
) or die $DBI::errstr;

if ($rows == 0) { # no record updated, so we insert new one
  $dbh->do(qq~
    insert into mytable values ( ?, ? )
    ~, undef, $mykey, $myvalue
  ) or die $DBI::errstr;
}