second.txt
9,/vol/elephant,fourfeet
1999,/vol/dolphin,fish
10,/vol/cat,feline
1111,/vol/goldfish,fish
2222,/vol/spider,arachnid
5555,/vol/camel,dromedary

<code>
#!/usr/bin/perl
use strict;
use warnings;

sub get_file {
  open my $FILE, '<', shift or die $!;
  return map {$_ => $_} <$FILE>;
return map {$_->[0] => $_->[1]} <$FILE>;
}

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

{
  no warnings 'uninitialized';
  print "$_\n" for grep {$_} @a{keys %b};
}