my $sth = $dbh->prepare( 
'select * from mytable order by col_3' 
) or die DBI->errstr;
$sth->execute() or die DBI::errstr;
while( my $aref = $sth->fetch ) {
    print "@$aref";
}

##</code><code>##

my $sth = $dbh->prepare( 
'select * from mytable order by ?' 
) or die DBI->errstr;
$sth->execute( 'col_3' ) or die DBI::errstr;
while( my $aref = $sth->fetch ) {
    print "@$aref";
}