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my @a = ( 3, 1, 4, 1, 5 );
my $N = @a;
...
my $aref = \@a;
$refsize = @$aref; # or @{$aref};
say $refsize;

my @twoD = ( [ 1, 2, 3 ],
             [ 4, 5, 6 ],
...
for my $row ( @twoD ) {
   print "row has " . scalar @$row . " elements\n";
}