This article explains and compares several @(posedge clk) statements.

Compare

always @(posedge clk) begin
    // codes block 1
end

always begin
    @(posedge clk);
    // codes block 2
end

@(posedge clk) begin
    // codes block 3
end

@(posedge clk);
// codes block 4

The first snippet: Execute code block whenever at (posedge clk).

The second snippet: Only used in procedural block (in testbench). It behaves the same as the first snippet.

The third snippet: Only used in procedural block (in testbench). Block the following code block until (posedge clk). When the posedge arrives, execute code block.

The fourth snippet: It behaves the same as the third snippet.

References