So to get both pieces of information I have to call unpack twice? 
my $len = unpack 'C/a*', $packed;
my ($data) = unpack 'C/a*', $packed;

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No... you might have missed that spot in the docs that say "back up a byte". I will use it to effectively unpack the same byte twice, with one unpack(), and one template. 
print join ":", unpack 'CXC/a*', $packed;

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resulting in 
19:the quick brown fox

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 Another option, though less user-friendly, would be to use the '@' template, which you can use to reposition the current pointer — think of it as a form of seek(). 
print join ":", unpack 'C@0C/a*', $packed;

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the "0" being the absolute position — here, the start of the string. Actually, it turns out that if you omit it, zero is used as a default.

The user-unfriendlyness is in the fact that you need to know the exact absolute position to seek to — making it impractical to encorporate it in the middle of a larger template — and not a relative one. Well, that's what the "X" is for, if you want to go backwards, and "x", to go forward. 

print join ":", unpack 'C@C/a*X15a5x7a*', $packed;

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which results in: 
19:the quick brown fox:quick:fox

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In reply to Re: unpack 'C/a*' and context weirdness by bart
in thread unpack 'C/a*' and context weirdness by BrowserUk

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