I have not been able to come up with a way to do it in a single unpack, but if I post my solution maybe someone will come up with something better:

# Arguments are datum length (5, 6, 7, or 8) and stream data (whose
# length must be a multiple of the datum length)
sub unpack_stream {
    my ( $length, $stream ) = @_;
    $length ||= 5;
    $length >= 5
    and $length <= 8
    or die "Length $length must be between 5 and 8 inclusive\n";
    my $pad = pack 'C*', ( 0 ) x ( 8 - $length );
    return map { unpack 'Q<', "$_$pad" } unpack "(a$length)*", $stream
+;
}
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