As well as what Abigail said, how are you reading your log file? If you're doing something like ...

@log_entries = <LOGFILE>;
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then you'll be slurping the whole file into memory. Better to do ...

while(<LOGFILE>) {
    # stuff to process this single log entry
}
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This:

$maillist{$hostqid}=$mail;
$msgid=${maillist{$hostqid}}->msgid;
delete $maillist{$hostqid};
$maillist{$hostqid}=$nmail;
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also looks weird. You assign to $maillist{$hostqid}, then use it, then immediately delete the hash entry, then immediately assign a new value to it.