Create a two-dimensional array at runtime

Line 1: The input parse doesn't care how you separate the dimensions as long as there are two distinct numbers.

Line 2: The list replication operator xx will automatically thunkify its left side so this produces new subarrays for each replication.

Line 3: Subscripting with a closure automatically passes the size of the dimension to the closure, so we pick an appropriate random index on each level.

Line 4: Print each line of the array.

my ($major,$minor) = prompt("Dimensions? ").comb(/\d+/);
my @array = [ '@' xx $minor ] xx $major;
@array[ *.rand ][ *.rand ] = ' ';
.say for @array;

Typical run:

Dimensions? 5x35
[@ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @]
[@ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @]
[@ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @]
[@ @ @ @ @ @ @ @ @ @   @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @]
[@ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @ @]

The most recent versions of Rakudo have preliminary support for 'shaped arrays'. Natively shaped arrays are a flexible feature for declaring typed, potentially multi-dimensional arrays, potentially with pre-defined dimensions. They will make memory-efficient matrix storage and matrix operations possible.

my ($major,$minor) = +«prompt("Dimensions? ").comb(/\d+/);
my Int @array[$major;$minor] = (7 xx $minor ) xx $major;
@array[$major div 2;$minor div 2] = 42;
say @array;

Typical run:

Dimensions? 3 x 10
[[7 7 7 7 7 7 7 7 7 7] [7 7 7 7 7 42 7 7 7 7] [7 7 7 7 7 7 7 7 7 7]]