Thank you everyone, very much appreciate all the assistance. Turns out the code in fact wasn't working correctly after all.

Especially thanks to 'Loops' for the suggestion of using a hash to track exits. I had been worrying about loading the whole dataset into program memory and this suggestion resolved that.

Finally ended up with the following - probably not perfect but it is definitely working and thoroughly tested. It does seem to have been an awful lot of effort for a relatively trivial program, but I suppose you have to expect that when you're trying to learn a new language.

#! /usr/bin/perl

use DBI;

$dbc= DBI->connect('dbi:Pg:dbname=redacted;host=localhost', 'redacted'
+, '', { AutoCommit=>1, RaiseError=>1, PrintError=>0 });

$sth = $dbc->prepare
   (" 
        SELECT 
            TO_CHAR(SN_EVENTS.CREATED, 'YYYY-MM-DD HH24:MI:SS') AS DAT
+ETIMED,
            SN_CAMERAS.TYPE AS DIRECTION, 
            sefs.ocr_1 as OCR
        FROM 
            SN_EVENTS
                INNER JOIN sn_events_fusions sefs ON sefs.fk_event = s
+n_events.pk_event
                INNER JOIN SN_CAMERAS ON SN_CAMERAS.PK_CAMERA = SN_EVE
+NTS.FK_CAMERA 
                INNER JOIN SN_RULES ON SN_RULES.PK_RULE = SN_EVENTS.FK
+_RULE 

        WHERE SN_CAMERAS.TYPE = 'entrance' OR SN_CAMERAS.TYPE = 'exit'
            AND SN_RULES.key = 'plate-recognition'
            AND SN_EVENTS.FK_EVENT_STATUS = 1 

        ORDER BY SN_EVENTS.CREATED DESC
    ");

$sth->execute or die $sth->errstr;

while (@event = $sth->fetchrow_array()){
    if ($event[1] eq "exit"){
        $exited{$event[2]}=1;
    }
    if (not $exited{$event[2]}){
        push @recordset, [@event];
    }
};

$sth->finish;

reverse(@recordset);

for $row ( @recordset ){
    print "@$row\n";
}
[download]