It doesn't give me a single format string that I can use to convert back and forth between "records" and "fields".

The "problem" is that you cannot have a single conversion that produces binary numeric data for internal math use, and also produces asciified hex string for display purposes.

It's the same as asking for a single printf format that will display an number as decimal and hex. It cannot be done.

The solution is to have two templates: 1 for internal packing and unpacking:

$templ = 'A4 v d';;

@fields = ( 'fred', 12345, 123.456 );;

$record = pack $templ, @fields;;

print unpack $templ, $record;;
fred 12345 123.456
[download]

And another for unpack a record for display purposes:

$templ2 = 'A4 H4 H16';;

print unpack $templ2, $record;;
fred 3930 77be9f1a2fdd5e40
[download]

There is simply no way to implement a single template that would server both purposes.