I cannot install PDL at the moment because I'm not on "my" machine, but I was wondering why this should lead to a syntax error.

~/sviluppo/perl> cat prova.pl
#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
my $mat = [
           [3.4603, 1.7529, 1.6990,2.1181],
           [ 2.5459,2.5615,2.2525,2.3606],
           [ 2.3606, 2.2515, 2.5628,2.5459],
           [ 2.1181,1.6982,1.7537,3.4603],  # <-- trailing comma
           ];
print Dumper($mat);
~/sviluppo/perl> perl prova.pl
$VAR1 = [
          [
            '3.4603',
            '1.7529',
            '1.699',
            '2.1181'
          ],
          [
            '2.5459',
            '2.5615',
            '2.2525',
            '2.3606'
          ],
          [
            '2.3606',
            '2.2515',
            '2.5628',
            '2.5459'
          ],
          [
            '2.1181',
            '1.6982',
            '1.7537',
            '3.4603'
          ]
        ];
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Neither errors nor warnings. I think that the list constructor doesn't complain about trailing "hanging" commas because you're able to cut-and-paste lines (like in this example) without the need to treat the last line differently.

Is it different when PDL comes in?