Erm..

for my $i (0..20) {
    vec( $bitstring, $offset++, 4 ) = $i ;
}
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That sets consecutive nybbles (half-bytes) to 0,1,2,3 .. etc.

my $bits = unpack("A4*", $bitstring);
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This "unpacks" an ascii string of 4 bytes (the * at the end doesn't do anything since you didn't give a format letter). In other words, it just returns the first 4 bytes of $bitstring without converting them in any way.

I don't think there's an unpack template for nybbles. If you want to see what you put in $bitstring, you might want to use the bit string template: "b*":

print unpack("b*",$bitstring);
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output (broken up for readability):

0000100001001100001010
1001101110000110010101
1101001110110111111100
0010000100110000100000
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or hex:

print unpack("h*",$bitstring);
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output:

0123456789abcdef012340
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