#!/usr/bin/perl -l

# http://perlmonks.org/?node_id=1209857

use strict;
use warnings;

$_ = <<END; # starting conditions
John apples,cherries,pears blacksmith,fisherman,programmer
Patrick apples,cherries,pears blacksmith,fisherman,programmer
Edward apples,cherries,pears blacksmith,fisherman,programmer
END

my $prev;
do
  {
  print;
  $prev = $_;

  s/^John \S*\Kpears//m;               # John doesn't like pears
  s/^Patrick \S+ \K\S+/blacksmith/m;   # Patrick is a blacksmith
  s/^Edward \S+ \S*\Kfisherman//m;     # Edward isn't a fisherman
  s/\S+(?= programmer$)/cherries/m;    # The programmer likes cherries

  s/,+\K,|\B,+|,+\B//g;                # cleanup extra commas

  for my $col (1 .. tr/ // / tr/\n// ) # for each column
    {
    for my $one ( /^(?:\S+ ){$col}(\w+)\s/gm ) # find each single one
      {
      s/^(?:\S+ ){$col}(?:\K$one,|\S+\K,$one\b)//gm; # delete in other rows
      }
    }
  } until $_ eq $prev;