I'd use a hash, because you refer to sections of the record by name. Something like the following might work for reading in records, though it's untested.

my @messages;

# loop through directory
{
    local ($/, *INPUT);
    open(INPUT, $dirname . $file) or die;
    my %record = split(/<(SENDER|TO|FROM|MESSAGE)>/, <INPUT>);
    push @messages, \%record;
}
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