my $file_A = read_file( "file_A" );
my $file_B = read_file( "file_B" );

foreach (keys %$file_A) {
  delete $file_A->{ $_ } unless exists( $file_B->{ $_ } );
}

sub read_file {
  my $file = shift;
  open FILE, $file or die "Can't open '$file' for reading $!";
  my @file = <FILE>;
  close FILE;

  my $hoa = {};
  foreach ( @file ) {
    my (@fields) = split /:/;
    my $key = $fields[0];
    $hoa->{ $key } = [ @fields ];
  }
  return $hoa;
}

##</code><code>##


sub change_field_to_check {
  my $hoa = shift;
  my $field = shift;
  $field -= 1;

  my $new_arrangement;

  foreach (keys %$hoa) {
    $new_arrangement->{ $hoa->{ $_ }[ $field ] } = $hoa->{ $_ };
  }

  return $new_arrangement;
}

$file_A = change_field_to_check( $file_A, 4 ); # Field 4
$file_B = change_field_to_check( $file_B, 4 ); # Field 4

foreach (keys %$file_A) {
  delete $file_A->{ $_ } unless exists( $file_B->{ $_ } );
}