在perl中，我使用dbi概念连接了数据库。数据库连接和select表查询在.pl文件中工作正常。但是我已经在.pm文件中运行了dbi数据库连接代码。没用了。
请检查代码。
sample.pl（工作正常）

use DBI;
my $driver = "mysql"; 
    my $database = "marketplace_perl";
    my $dsn = "DBI:$driver:database=$database";
    my $userid = "root";
    my $password = "root";
    my $dbh = DBI->connect($dsn, $userid, $password );
    my $dbh=connect_db();
        my $sth = $dbh->prepare("SELECT UserEmail,UserPassword FROM pj_user where UserEmail=? and UserPassword=?");

        $sth->execute($UserEmail,$UserPassword);

marketplace.pm（不工作）

package Marketplace;
use DBI;
    sub connect_db {
        my $driver = "mysql"; 
        my $database = "marketplace_perl";
        my $dsn = "DBI:$driver:database=$database";
        my $userid = "root";
        my $password = "root";
        my $dbh = DBI->connect($dsn, $userid, $password );
       return $dbh;
    }

    sub login_marketplace { 

    my $dbh=connect_db();
            my $sth = $dbh->prepare("SELECT UserEmail,UserPassword FROM pj_user where UserEmail=? and UserPassword=?");

            $sth->execute($UserEmail,$UserPassword);
            my $User_count=$sth->rows;
    return $User_count

    }

它返回错误消息“failed to access class（marketplace）：can't locate dbi.pm in@inc（you may need to install the dbi module）”
请让我知道如何解决数据库问题。