I do not understand your output in this case 'vzI&#..'

The last of the 4 characters, the ═ bit, is just a side effect of pasting console output into PMs code block.

Somewhere along the line, some characters with the high-bit set (ie. chars greater than 127) get converted into unicode html escape sequences.

Outside of a code block that looks like this: ═, which is exactly how it appeared on my console prior to C&P.

Is this behaviour unique to C?

No, it also applies to 'c' & 'W'.

As for documentation: for unpack, all it basically says is that it is the reverse of pack. Look at the docs for the latter, and think about the logical reverse.

But more than any other piece of Perl, the *only* way to understand pack & unpack, is to get yourself a REPL and sit and play.