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 ...
}