Here's what it would look like if you actually created a 2d array:

sub perl_routine
{
    my @array = map { pack "I*", (0) x $x_sz } 1..$y_sz;
    my $array = pack "P"x@array, @array;
    c_function( $array, $y_sz, $x_sz );
}

void c_function( char *arg, int y_sz, int x_sz )
{
    int ** const table = ( int ** )arg;
    int y, x;

    for ( y = 0; i < y_sz, y++ ) {
        for ( x = 0; x < x_sz, x++ ) {
            ++table[y][x];
        }
    }
}
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It's odd that P* doesn't work