That is a very cumbersome way to do things, as well as being a performance penalty rebuilding the template every time you use it.

The basic problem is that you are setting up a (series of) variable(s), when you should be using a constant!

Yes, the template might need to change in the future, but it won't be changing at runtime, so it is a constant. Both sets of lines can be replaced by a single line (and a comment):

use constant PACK_TEMPLATE = 'H4 b7 b1 b1 b1 b6 H2 A8 H4';
=comment 
'H4';# 16 bits     Msg ID                          
'b7';# 7 bits      Frag Seq                        
'b'; # 1 bit       More Frags flag                 
'b'; # 1 bit       Unit ID Present                 
'b'; # 1 bit       CRC Present                     
'b6'; # 6 bits Unused
'H2'; # 8 bits     Frag Length  
'A8'; # if present, unit ID is 8 chars
'H4'; # if present, CRC is a 16 bit word                      
=cut
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Now, you just use PACK_TEMPLATE (give it a better name appropriate to your application), whereever you need it. It will be private to the package/class you declare it in, so there are no action at distance concerns.