Super Mario All Stars (All-Stars): The Official Companion Guide

As Super Mario or Fire Flower Mario you must go to the pipe right at the end of the level and jump on it. Break the block above/left and stand on the left edge of the pipe as far over as you can without falling off. Crouch then jump backwards as far right as you can. If you get this right Mario will slide through the pipe and bricks. Go over to the warp zone area and enter the middle pipe. This pipe then warps you straight to world 5, level 1.

Another way of getting to 5-1 is by doing a very similar thing in world 4-2 - thats actually meant to be there though, the warp from 1-2 is a glitch. Once you have got to 5-1 from 1-2, you can also use either of the other two pipes to access Minus World -1.