Ok, this one actually gives the output you asked by ignoring order.

use strict;
use warnings;

my %sets;
my %owner;
while (<DATA>) {
   my ($p, $q) = sort split;

   next if $p eq $q;

   $sets{ $owner{$p} ||= $p }{$p} = 1;
   $sets{ $owner{$q} ||= $q }{$q} = 1;

   %{ $sets{$p} } = (
      %{ delete( $sets{ $owner{$p} } ) },
      %{ delete( $sets{ $owner{$q} } ) },
   );

   $owner{$p} = $p;
   $owner{$q} = $p;
}

{
   local $\ = "\n";
   local $, = "\t";
   print(keys(%$_)) for values(%sets);
}

__DATA__
read      book
eat       apple
book      novel
apple     banana
play      football
tennis    football
novel     mazagine
[download]

eat     banana  apple
read    book    mazagine        novel
tennis  play    football
[download]