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open(my $fp, '<:raw', $filename) or die "Couldn't open $filename: $!";

read($fp, my $type_id, 4);
if ($type_id eq 'FORM') {
  ...
}

read($fp, my $buffer, 4);
my $type_id = unpack('A4', $buffer);
if ($type_id eq 'FORM') {
  ...
}