my $ST = $DB->prepare("select foo, bar from my_table "
                     ."where col1 < '$abc' or (col1 > '$def' and col3 < $fgh)");
$ST->execute();

##</code><code>##

my $ST = $DB->prepare("select foo, bar from table where col1 < ? or (col1 > ? and col3 < ?)");
$ST->execute($abc, $def, $fgh);

##</code><code>##

select foo, bar from my_table where col1 < '';
drop table my_table
; --' or (col1 > 'def_val' and col3 < fgh_val)