Keywise diff of two .csv data files

# Compare the values of the second column of two .csv files
# Using the first column as key
# Ignoring keys which only exist in second file
# Result .csv contains new and old values for each key,
# and additionally the percentual in- or decrease in a fourth column

use constant {
  BASE_DIR => "G:\\MY_BASE_DIRECTORY\\",
  SEP      => "\t"   # Separator used in .csv files
};

use Text::CSV qw( csv );

my $fnew = read_csv( "kw-49-2020.csv" );
my $fold = read_csv( "kw-47-2020.csv" );

while (my ($key, $new) = each %$fnew) {

  print "$key".SEP."$new";

  if (my $old = $fold->{$key}) {
    print SEP."$old";
    my $trend = 100 * ( $new - $old ) / $old;
    printf( SEP."%.2f",$trend );
  }
  print "\n";
}

sub read_csv {

  my $aoa = csv (in => BASE_DIR . shift, sep_char=> SEP );
  my %stats = ();
  for my $line (@$aoa) {
    my @record = @$line;
    my $key = ( $record[0] =~ s/^\s*(\w+)\s*/$1/r );
    $stats{$key} = $record[1];
  }

  return \%stats;

}