In C or Perl I'd break the string into 3 byte chunk and extract the 6 bit bytes using bitmasks. Something like this:

    my $x = pack 'V', 1234567;

    my $mask0to5   = 2**6 - 1;
    my $mask6to11  = $mask0to5 << 6;
    my $mask12to18 = $mask0to5 << 12;
    my $mask18to24 = $mask0to5 << 18;

    sub unpack_6bit_bytes {

        my $byte_str = unpack 'V', $_[0];
        my $byte_1   =   $byte_str & $mask0to5;
        my $byte_2   = ( $byte_str & $mask6to11 )  >> 6;
        my $byte_3   = ( $byte_str & $mask12to18 ) >> 12;
        my $byte_4   = ( $byte_str & $mask18to24 ) >> 18;

        return ( $byte_1, $byte_2, $byte_3, $byte_4 );
    }

    print join ' ', unpack_6bit_bytes($x);
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This is rough code to demonstrate the general method and only quickly tested.

--
John.