[sguav]

always @(posedge clk or negedge clk)

[acadiasprings]

This is the correct syntax, though many FPGA architectures don't support double edge registers

[sguav]

True, I directly answered the syntax-related question, but talking about implementation this would be a bad description - be it FPGA or ASIC.

As the parent's-parent comment said in other words, think circuit first.