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# prepare the sql once
my $sth = $dbh->prepare(q/select column1 from mytable where column2 = 
+?/);
...
   $sth->execute($my_key_into_table);
   my ($column1) = $sth->fetchrow;  
}

my $hashref = $dbh->selectall_hashref(q/select column1,key_column from
+ mytable/, 'key_column');