use Modern::Perl;
use Text::CSV;

my $csv = Text::CSV->new() or die "Cannot use CSV: " . Text::CSV->error_diag();

open my $fh_in,  '<', "test.csv"     or die "test.csv: $!";
open my $fh_out, '>', "test_new.csv" or die "test_new.csv: $!";

while ( my $line = <$fh_in> ) {
    my $status = $csv->parse($line);
    my ( $field1, $field2, $field3, $field4 ) = $csv->fields();
    $status = $csv->combine( $field3, $field2, $field1, $field4 );
    my $output_string = $csv->string();
    print $fh_out $output_string;
}