echo abABCDCBAdeXYZYXfg1221 | perl -Mv5.14 -nle'
   /
      (
         (.)
         (?: (?-2) | .? )
         \g-1
      )
      (?{ say $1 })
      (*FAIL)
   /xs
'

##</code><code>##

echo abABCDCBAdeXYZYXfg1221 | perl -Mv5.14 -nle'
   /
      ( (?&PALINDROME) )
      (?{ say $1 })
      (*FAIL)

      (?(DEFINE)
         (?<PALINDROME>
            (.)
            (?: (?&PALINDROME) | .? )
            \g-1
         )
      )
   /xs
'