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$ perl -Mre=debug -E'$x=qr/[X0-9a-f]/i; $hr=qr/$x+ - $x+/x;'
Compiling REx "[X0-9a-f]"
...
Freeing REx: "(?^ui:[X0-9a-f])+ - (?^ui:[X0-9a-f])+"
Freeing REx: "[X0-9a-f]"
$

$ 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:
...
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])+"
$