/vol/cat,feline
/vol/dog,canine
/vol/cat,feline
/vol/cat,feline
/vol/amphibian,FROG
/vol/amphibian,FROG

##</code><code>##

9,/vol/elephant,fourfeet
1999,/vol/dolphin,fish
10,/vol/cat,feline
1111,/vol/goldfish,fish
2222,/vol/spider,arachnid
5555,/vol/camel,dromedary
3333,/vol/wolf,canine

##</code><code>##

use strict;

sub get_animal {
  open my $FILE, '<', shift or die $!;
  return map {chop; $_ => $_} <$FILE>;
}

my %a = get_animal '/tmp/first.txt';
my %b = get_animal '/tmp/second.txt';

{
  
  print "$_\n" for grep {$_} @a{keys %b};
}