This snipplet just prints common keys in the two files file1.txt and file2.txt:

se strict;
use warnings;

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

my %a = get_file 'file1.txt';
my %b = get_file 'file2.txt';

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

The above can easilly be done using sort, uniq and grep.