my $module_dbh;

sub
DBModule_Start
{
    unless ($module_dbh) {
       $module_dbh = DBI->connect("DBI:mysql:database=db_name", "user_name", "password");
    }
    return $module_dbh;
}

sub
DBModule_Sub1
{
    $module_dbh->do{'BLAH BLAH BLAH'} or die $module_dbh->errstr;
}

sub
DBModule_Finish
{
    $module_dbh->disconnect;
}