Those eval make no sense and you're missing the /x. You simply want

my $nfw_oang = qr%    
    (?!\p{IsOang}(?!
        (?:\p{InThaiFinCons}){1,2}    
        (?![\p{InThaiCompVowel}\p{InThaiPostVowel}\p{InThaiTone}])
        (?:\p{InThaiMute})
    ))
%x;
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And given that (?![\p{X}\p{Y}\p{Z}])\p{W} can be written as (?[ \p{W} - [\p{X}\p{Y}\p{Z}] ]) or (?[ \p{W} - \p{X} - \p{Y} - \p{Z} ]), you could use

my $nfw_oang = qr%    
   (?!
      \p{IsOang}
      (?! (?:\p{InThaiFinCons}){1,2}
          (?[
             \p{InThaiMute}
             - \p{InThaiCompVowel}
             - \p{InThaiPostVowel}
             - \p{InThaiTone}
          ])
      )
   )
%x;
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That said, you might want to look into (?(DEFINE)...).

/
   ...
   (?&NFW_OANG)
   ...
   (?&INITIAL_CONSONANT)
   ...

   (?(DEFINE)
      # NOT FOLLOWED BY O-ANG WITHOUT A THAI MUTE/CONSONANT ENDING
      (?<NFW_OANG> (?! (?&OANG) ) )

      # O-ANG WITHOUT A THAI MUTE/CONSONANT ENDING
      (?<OANG>
         \p{IsOang}
         (?! (?:\p{InThaiFinCons}){1,2}
             (?[
                \p{InThaiMute}
                - \p{InThaiCompVowel}
                - \p{InThaiPostVowel}
                - \p{InThaiTone}
             ])
         )
      )

      # INITIAL CONSONANT(S)
      (?<INITIAL_CONSONANT>
         (?: \p{InThaiDualC1} \p{InThaiDualC2}
         |   \p{InThaiCons}
         )
      )
/x
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