'1234'
/(?<P>(?&V))(?<Q>.)(?(DEFINE)(?<V>...))/

&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m    (?<P>           &#8592;[0m  &#8592;[36;40m    The start of a named capturing block (also $1)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m      (?&V)         &#8592;[0m  &#8592;[36;40m      Match a call to the subpattern named <V>&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m    )               &#8592;[0m  &#8592;[36;40m    The end of the named capturing block&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m    (?<Q>           &#8592;[0m  &#8592;[36;40m    The start of a named capturing block (also $2)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m      .             &#8592;[0m  &#8592;[36;40m      Match any character (except newline)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m    )               &#8592;[0m  &#8592;[36;40m    The end of the named capturing block&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m    (?(DEFINE)      &#8592;[0m  &#8592;[36;40m    The start of a definition block (skipped during matching)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m      (?<V>         &#8592;[0m  &#8592;[36;40m      The start of a named capturing block (also $3)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m        .           &#8592;[0m  &#8592;[36;40m        Match any character (except newline)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m        .           &#8592;[0m  &#8592;[36;40m        Match any character (except newline)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m        .           &#8592;[0m  &#8592;[36;40m        Match any character (except newline)&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m      )             &#8592;[0m  &#8592;[36;40m      The end of the named capturing block&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m&#8592;[37;40m    )               &#8592;[0m  &#8592;[36;40m    The end of definition block&#8592;[0m
&#8592;[34;40;4m
&#8592;[0m

##</code><code>##

$_=1234;
m{
  (?<P>(?&V))  # match <V> and save to $+{P} 
  (?<Q> .)     # match <Q> and save to $+{Q}

# this can be saved in $v_definition = qr//
  (?(DEFINE)
    (?<V> ...) # <V> aka (?&V) is three chars
  )
}xm;

##</code><code>##

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

    (?&V)         #      Match a call to the subpattern named <V>

  )               #    The end of the named capturing block

  (?<Q>           #    The start of a named capturing block (also $2)

    .             #      Match any character (except newline)

  )               #    The end of the named capturing block

  (?(DEFINE)      #    The start of a definition block (skipped during matching)

    (?<V>         #      The start of a named capturing block (also $3)

      .           #        Match any character (except newline)

      .           #        Match any character (except newline)

      .           #        Match any character (except newline)

    )             #      The end of the named capturing block

  )               #    The end of definition block
}x