Hm?

Big-endian:

sub pack24{ substr( pack('l>', $_[0]), 1) };;
sub unpack24{ unpack('l>', "$_[0]\0") };;

print "$_: ", unpack24( pack24( $_ ) ) 
    for ( -8388608, -8388607, -2, -1, 0, 1, 2, 8388606, 8388607 );;
-8388608:  -2147483648
-8388607:  -2147483392
-2:  -512
-1:  -256
0:  0
1:  256
2:  512
8388606:  2147483136
8388607:  2147483392
[download]

Little-endian:

sub pack24{ substr( pack('l<', $_[0]), 1) };;
sub unpack24{ unpack('l<', "$_[0]\0") };;

print "$_: ", unpack24( pack24( $_ ) ) 
    for ( -8388608, -8388607, -2, -1, 0, 1, 2, 8388606, 8388607 );;
-8388608:  16744448
-8388607:  16744448
-2:  16777215
-1:  16777215
0:  0
1:  0
2:  0
8388606:  32767
8388607:  32767
[download]