pack("Yv", -1);      # yields "ffff"

I thought that is what you meant by functional composition, but I wanted to be sure.

The problem is, abuttal of template characters is already allowed. pack 'NQ', ... says pack the first argument as a unsigned dword (in network order), and the second as an unsigned quadword. And it is entirely equivalent to pack 'N Q', ... and pack 'N1Q1, ....

In other words, abuttal has no significance. You are suggesting making it significant. Besides that it could break existing code, whilst it may possible to code Your Y character to expect and handle abuttal to be significant, the human eye/brain is less reliable.

The beauty of my suggestion is that it is a natural extension of existing implemented syntax.