Please use use strict. Not using it hides errors, including the one you made.

use strict;
use warnings;

my @a=([0,1,2]);
my @b=([-1,-2,-3]);

unshift @a,1;    # Creates an element with the value "1".
$a[0][0]=4;      # Changes element 0 of the array named by $a[0].
$a[0][1]=5;      # Changes element 1 of the array named by $a[0].
$a[0][2]=6;      # Changes element 2 of the array named by $a[0].

unshift (@b,1);  # Creates an element with the value "1".
$b[0][0]=7;      # Changes element 0 of the array named by $b[0].
$b[0][1]=8;      # Changes element 1 of the array named by $b[0].
$b[0][2]=9;      # Changes element 2 of the array named by $b[0].

print $a[0][2];  # Print element 2 of the array named by $a[0].
print $a[1][2];  # Print element 2 of the array ref'ed by $a[1].
print $b[0][2];  # Print element 2 of the array named by $b[0].
print $b[1][2];  # Print element 2 of the array ref'ed by $b[1].
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Can't use string ("1") as an ARRAY ref while "strict refs" in use at s
+cript.pl line 8.
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If you want to add a new row, use unshift @a, [ ];.

unshift @a, [ ];
$a[0][0]=4;
$a[0][1]=5;
$a[0][2]=6;
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or

unshift @a, [ 4, 5, 6 ];
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use strict;
use warnings;

use Data::Dumper;

my @arrA = ( [0, 1, 2] );
unshift @arrA, 1;

print Data::Dumper->Dump([\@arrA], [q{*arrA}]);

shift @arrA;
unshift @arrA, [1];

print Data::Dumper->Dump([\@arrA], [q{*arrA}]);
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This code produces the following.

@arrA = (
          1,
          [
            0,
            1,
            2
          ]
        );
@arrA = (
          [
            1
          ],
          [
            0,
            1,
            2
          ]
        );
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I hope this is useful.

Cheers,

JohnGG

You may be surprised to learn that @a=([0,1,2]); creates an array with one element in it. The first scalar in @a is an arrayref. that array ref has three elements.

You probably want to do  unshift @a, [1]. Now @a is an array with two elements in it. Both are scalary array refs.

See perlref for all the magic. I used to read that page quite often (probably twice a year or more).

To be pedantic, Perl doesn't have multi-dimensional arrays; it has arrays where the elements can be references to other arrays (or hashes or any other data structure), so pop and unshift will remove an array element, which would happen to be a reference.

so pop and unshift will remove an array element

I'm quite sure you meant to say pop and shift.

-David