I guess I'll have to leave off the last two parts which in this application are optional, and add them to PACK_TEMPLATE programatically after the length field

Personally, I'd just create 2 or 3 constants and select between them at runtime:

use constant T1 = '...';
use constant T2 = t1 . '...';
use constant T3 = T1 . '...';
# or 
use constant T3 = T2 . '...';

...

my $packed = pack ( T1, T2, T3 )[ condition() ], @bits;
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Wher condition() returns an integer 0 .. 2.