# prepare the sql once
my $sth = $dbh->prepare(q/select column1 from mytable where column2 = ?/);
#
while(something) {
   # assuming only one column and one row are returned
   $sth->execute($my_key_into_table);
   my ($column1) = $sth->fetchrow;  
}

##</code><code>##

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