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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[ ] creates an anonymous array and returns a reference.