How to use join?

OK, what you're doing here:

print join ",", @$_ for @input_list ;

Isn't working, because it's:

• iterating @input_list extracting each element into $_.
• Dereferencing $_ pretending it's an array @$_.

This is basically the same as trying to:

print join ( ",", @{"10"} );

Which makes no sense, and so doesn't work.

my $string = join ( ",", @input_list );
print $string; 

Will do the trick.

The thing that you're missing here I think, is this:

use Data::Dumper; 
my @x = qw/10 20 30 40/;
my @y = qw/60 70 8 90 10/;
my @input_list = (@x, @y);
print Dumper \@input_list;

Isn't generating a multi-dimensional list. It's a single dimensional one.

$VAR1 = [
          '10',
          '20',
          '30',
          '40',
          '60',
          '70',
          '8',
          '90',
          '10'
        ];

I suspect what you may want is:

my @x = qw/10 20 30 40/;
my @y = qw/60 70 8 90 10/;
my @input_list = (\@x, \@y);

Or perhaps:

my $x_ref = [ qw/10 20 30 40/ ];
my $y_ref = [ qw/60 70 8 90 10/ ];
my @input_list = ($x_ref, $y_ref );

Which makes @input_list:

$VAR1 = [
          [
            '10',
            '20',
            '30',
            '40'
          ],
          [
            '60',
            '70',
            '8',
            '90',
            '10'
          ]
        ];

Then your 'for' loop works:

print join (",", @$_),"\n" for @input_list ;

Because then, @input_list is actually 2 items - two array references that you can then dereference and join.

As a slight word of warning though - one of the gotchas that can occur when doing:

my @input_list = (\@x, \@y);

Because you're inserting references to @x and @y - if you reuse either of these, then you'll change the content of @input_list - which is why it's probably better to use the my @input_list = ( $x_ref, $y_ref );.