> Pretty sure they're not kept as nested objects.

That was my first intuition, and looks like we were right, the final program is the same for the flattened string. 

$ perl -Mre=debug -E'$x=qr/[X0-9a-f]/i; $hr=qr/$x+ - $x+/x;'
Compiling REx "[X0-9a-f]"
Final program:
   1: ANYOF[0-9A-FXa-fx] (11)
  11: END (0)
stclass ANYOF[0-9A-FXa-fx] minlen 1             
Compiling REx "(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+"
Final program:                                  
   1: PLUS (12)
   2:   ANYOF[0-9A-FXa-fx] (0)
  12: EXACT <-> (14)                           
  14: PLUS (25)                                
  15:   ANYOF[0-9A-FXa-fx] (0)
  25: END (0)
floating "-" at 1..9223372036854775807 (checking floating) stclass ANY
+OF[0-9A-FXa-fx] plus minlen 3
Freeing REx: "(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+"
Freeing REx: "[X0-9a-f]"
$

[download]

$ perl -Mre=debug -E'$hr=qr/(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+/x;' 
+                        
Compiling REx "(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+"                 
+                          
Final program:
   1: PLUS (12)
   2:   ANYOF[0-9A-FXa-fx] (0)                  
  12: EXACT <-> (14)                            
  14: PLUS (25)
  15:   ANYOF[0-9A-FXa-fx] (0)                 
  25: END (0)
floating "-" at 1..9223372036854775807 (checking floating) stclass ANY
+OF[0-9A-FXa-fx] plus minlen 3
Freeing REx: "(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+"
$

[download]

> the regexp would need to be compiled each time around the while loop

I think that's what /o was invented for

Update

So... Consequently any performance penalty coming with qr// could be mitigated by flattening it first and using the resulting string with /o modifier. 🤔

Cheers Rolf
(addicted to the Perl Programming Language :)
see Wikisyntax for the Monastery

In reply to Re^9: Reusing a complex regexp in multiple spots, escaping the regexp by LanX
in thread Reusing a complex regexp in multiple spots, escaping the regexp by ecm

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