epoptai's right -- there's no difference between pack 'c',$number and pack 'C',$number, ever. There is a difference when unpacking, of course.

What the translations 'c' and 'C' do when packing is to translate an integer to a corresponding character value. Your character values are most likely single-byte numbers. Each corresponds to a specific modulus of integers. In particular, the two most popular ways to assign representative integer values to the 256 bytes are 0..255 ("unsigned char") and -128..127 ("signed char").

But of course any integer value is congruent (modulo 256) to exactly one byte value, whichever of the 2 ranges you pick. So any integer has a unique translation to a byte. The reverse direction (unpack 'c',$str) is less single-valued: for instance, unpack 'C',(pack 'C',-1) == 255. Here unpack has to chose a specific range from which to pick an integer representing the byte value, and the two letter codes make a difference.

The same thing occurs for the other signed/unsigned letters for integer conversions in pack/unpack.