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As you said yourself, the code you've written may be stupidly over-complicated... but I'm assuming you're posting here not because of the code itself, but because the code produces an apparent bug with the Perl interpreter...

I found the debug switch use re qw{eval Debug ALL};, gives the extra output below:

Compiling REx "(?(?{$_ =~ |$regex (\w+ \w+)(?=\1)|})(?{s|$1||})|(?{s|$
+1 \w+"...
Starting first pass (sizing)
 >(?(?{$_ =~... |   1|  reg
                |    |    brnc
                |    |      piec
                |    |        atom
 >?(?{$_ =~ ... |    |          reg
 >?{$_ =~ |$... |   2|            reg
 >(?{s|$1||}... |   6|            brnc
                |    |              piec
                |    |                atom
 >?{s|$1||})... |    |                  reg
 >(?{s|$1 \w... |  12|            brnc
                |    |              piec
                |    |                atom
 >?{s|$1 \w+... |    |                  reg
Required size 16 nodes
Starting second pass (creation)
 >(?(?{$_ =~... |   1|  reg
                |    |    brnc
                |    |      piec
                |    |        atom
 >?(?{$_ =~ ... |    |          reg
 >?{$_ =~ |$... |   2|            reg
panic: top_env

C:\Temp\panic>
[download]

And this is where i get stuck on where to go next with the investigation... perhaps another monk could shed further light?

As you've noted from perldiag, a panic usually indicates a bug...
Next steps would be to distill the REx down to the most simple form that produces the issue. Then checkout if its a known fault... (can another more enlightened monk comment on where and how faults with Perl can be queried?)