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};
}