#!/usr/bin/perl

# https://perlmonks.org/?node_id=1221461

use strict;
use warnings;

my %mycorpus = (
    a => "date:#20180101# comment:#d1 d2 d3 d4 d5 d6#",
    b => "date:#20180101# comment:#b1 b2 b3 b4 b5 b6 b7# comment:#c1 c
+2 c3 c4 c5 c6#",
    c => "date:#20180101# comment:#d1 d2 d3 d4 d5 d6#",
);

open my $fh, '|-', 'sort | uniq -c' or die;

for ( values %mycorpus )
  {
  my ($date) = /date:#(\d+)#/;
  for ( /comment:#(.*?)#/g )
    {
    my @words = split;
    print $fh "$date @words[$_..$_+4]\n" for 0 .. @words - 5;
    }
  }

close $fh;
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Outputs:

      1 20180101 b1 b2 b3 b4 b5
      1 20180101 b2 b3 b4 b5 b6
      1 20180101 b3 b4 b5 b6 b7
      1 20180101 c1 c2 c3 c4 c5
      1 20180101 c2 c3 c4 c5 c6
      2 20180101 d1 d2 d3 d4 d5
      2 20180101 d2 d3 d4 d5 d6
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