For this particular case, you don't need a rep count because the whole thing is very well behaved.

print join(',', unpack("C/C* v*", $testinput)), "\n";
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Is this strictly an educational exercise, or do you have more complex data structures to interrogate? If the data is chunked in a different way, Counting Repetitions solves your trouble with parentheses, but this only works in the context where you get to pick how things are packed.

my $testinput = pack('C/a* a* a*', 
                     (pack 'Cvs', 1, 3, -5), 
                     (pack 'Cvs', 2, 4, -6)
                     );

print join(',', unpack("C/(Cvs)", $testinput)), "\n";
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