use Modern::Perl;
my %first_file;
{
    open my $FIRST, '<', 'first.txt' or die $!;
    while (<$FIRST>) {
        chomp;
        $first_file{$_} = '' ;
    }
}
my (@both, @first, @second);
{
    open my $SECOND, '<', 'second.txt' or die $!;
    while (<$SECOND>) {
        chomp;
        if (exists $first_file{$_}) {
            push @both, $_;
            delete $first_file{$_};
        }
        else {
            push @second, $_;
        }
    }
}
@first = keys %first_file;