my @files = qw( foo bar bat );
@files = byvalue( @files );
byreference( \@files );


sub add_txt_by_value {
    my @array = @_;

    foreach ( @array ) {
        $_ .= ".txt";
    }
    return @array;
}

sub add_txt_by_reference {
    my $arrayref = shift;
    
    foreach ( @$arrayref ) {
       $_ .= ".txt";
    }
}