Best way to sort these strings ? recursive way ? use Parse::RecDescent?

I don't see anything recursive about it and this is certainly not a job for Parse::RecDescent.

Have you actually tried anything here? Got code?

I'd approach this by building a data structure from all the input before outputting it. Knowing nothing more about your input than the sample you've provided, I'd use a structure that looked like this when complete:

{
  'EX1J' => { 
    'desc' => 'JOB1' 
    'pred' => [],
  },
  'EX2J' => { 
    'desc' => 'JOB2'
    'pred' => [], 
   },
  'EX3J' => { 
    'desc' => 'JOB3'
    'pred' => ['EX1J'], 
   },
  'EX4J' => { 
    'desc' => 'JOB3'
    'pred' => ['EX1J', 'EX2J'], 
   },
}
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-sauoq
"My two cents aren't worth a dime.";