I cannot say if you should query your database for all possible results and store them in a hash or query them one at a time as you find them. Apart from anything else I've no idea how big this dataset may be. However you do either:

# 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;  
}
[download]

or

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