use strict;
use warnings;

my %assocs;
while (<DATA>) {
   my ($k, $v) = split;
   $assocs{$k} = $v;
}

my %roots;
$roots{$_} = 1 for keys   %assocs;
$roots{$_} = 0 for values %assocs;

my @chains;
for my $root (grep $roots{$_}, keys %roots) {
   my $node = $root;
   my @chain;
   while (defined($node)) {
      push @chain, $node;
      $node = $assocs{$node};
   }
   push @chains, \@chain;
}

{
   local $\ = "\n";
   local $, = "\t";
   print @$_ for @chains;
}

__DATA__
read      book
eat       apple
book      novel
apple     banana
play      football
tennis    football
novel     mazagine