All of the above suggestions are excellent. Let me give you the most basic form of a generic solution. This is probably not the best solution, but hopefully seeing some code will whet your mind a bit. I've left out gobs of error handling, since you said you are already familiar with the basics of DBI.

use strict; use warnings;
use DBI;

#replace these with the correct connect strings, of course
my $usage_db  = DBI->connect('dbi:ODBC:usage','','');
my $detail_db = DBI->connect('dbi:ODBC:detail','','');

my $usage_st = $usage_db->prepare(q'
  SELECT uid,description,err_code FROM errors
  WHERE err_code = ?
');

my $detail_st = $detail_db->prepare(q'
  SELECT * FROM error_details
  WHERE uid = ?
');

#- CORE -#   

   my $fatals = get_general('FATAL'); # get all fatal errors;
   foreach my $record (@$fatals) {
     my $uid = shift @$record;
     my $detail = get_detail($uid);
  
     #output result and its detail
     print join('||',$uid, @$record, @$detail), "\n";
   }

#- END CORE -#

# this sub finds all records with a given err_code
sub get_general {
   my $code = shift;
   $usage_st->execute($code);
   my $res = $sth->fetchall_arrayref;
   return $res;
}

# this sub gets the details for a given uid
sub get_detail {
   my $uid = shift;
   $detail_st->execute($uid);
   my $res = $sth->fetchall_arrayref;
   return $res;
}
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