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'1234'
/(?<P>(?&V))(?<Q>.)(?(DEFINE)(?<V>...))/
...
&#8592;[0m&#8592;[37;40m    )               &#8592;[0m  &#8592;[36;40m
+    The end of definition block&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m

$_=1234;
m{
  (?<P>(?&V))  # match <V> and save to $+{P} 
...
    (?<V> ...) # <V> aka (?&V) is three chars
  )
}xm;

m{
  (?<P>           #    The start of a named capturing block (also $1)

...

  )               #    The end of definition block
}x