use DBI;

my $dbh = DBI->connect( ... );  # get your database handle
my $sql = "select i.name, i._eid, i.num, i.val, m.code from ...".
          " where i.name = m.code and m.code = ?";  # "?" is a "placeholder"

my $sth = $dbh->prepare( $sql );

open( R, "<", $datafile_name ) or die "$datafile_name: $!\n";
while (<R>)
{
    chomp;
    my ( $ref ) = ( /^([^\t]+)/ );

    $sth->execute( $ref );  # value passed to execute() fills the placeholder

    # use one of the DBI methods for getting the query results...
}

##</code><code>##

"select ... from ... where i.name = m.code and m.code in (select ref from tmp)"

##</code><code>##

my %ref;
open( R, "<", $datafile_name );
while (<R>) {
    if ( /^([^\t]+)/ ) {
        $ref{$1} = undef;
    }
}

my $dbh = DBI->connect( ... );
my $sth = $dbh->prepare( "select m.code, ... from ... where i.name=m.code");
$sth->execute;
my $rows = $sth->fetchall_arrayref;

for my $r ( @$rows ) {
    my ( $mcode, ... ) = @$r;
    next unless exists( $ref{$mcode} );

    # do something ...
}