#!/usr/bin/perl -w
    use strict;

    use constant EXPENSE_FILE      => 'expense.dat';
    use constant EXPENSE_REPORT    => 'final_expense.txt';
    use constant FIELDS_PER_RECORD => 4;
    use constant REPORT_HEADER     => "Department,Total\n";

    my $expenses   = get_expense_data( EXPENSE_FILE );
    my $deptTotals = sum_expenses_by_department($expenses, FIELDS_PER_RECORD);
    write_report( $deptTotals, EXPENSE_REPORT, REPORT_HEADER );

    sub get_expense_data {
        my $file = shift;
        open IN_FILE, "<", $file or die "Cannot open $file for reading: $!";
        my @expenses = <IN_FILE>;
        close IN_FILE;
        return \@expenses;
    }

    sub sum_expenses_by_department {
        my ( $expenses, $fields_per_record ) = @_;
        my %deptTotals;

        foreach my $expense ( @$expenses ) {
            my %record;
            my @fields = qw/ dept empID amount desc /;
            @record{ @fields } = split /,/, $expense, $fields_per_record;
            if ( ! exists $deptTotals{ $record{ dept } } ) {
                $deptTotals{ $record{ dept } } = 0;
            }
            $deptTotals{ $record{ dept } } += $record{ amount };
        }
        return \%deptTotals;
    }

    sub write_report {
        my ( $report_data, $file, $header ) = @_;
        open OUT_FILE, ">", $file or die "Cannot open $file for writing: $!";

        print OUT_FILE $header;
        foreach ( sort keys %$report_data ) {
            print OUT_FILE "$dept,$report_data->{ $_ }\n";
        }
        close OUT_FILE;
    }