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my $sth = $dbh->prepare( 
'select * from mytable order by col_3' 
...
while( my $aref = $sth->fetch ) {
    print "@$aref";
}

my $sth = $dbh->prepare( 
'select * from mytable order by ?' 
...
while( my $aref = $sth->fetch ) {
    print "@$aref";
}