If liquid culture is done properly you should not need to add peroxide. If you are getting contamination in culture then refining the process to prohibit that will be more effective than using peroxide.

If you are experiencing contamination in the initial colonization phase with spore or spawn, consider your substrate. Is it high in nutrient content? If you are using rye/grain or compost to spore or spawn into with liquid culture, the extra nutrients in these substrates can be attractive for other unwanted guests at your most vulnerable point in the process. If you have rudimentary sterilization procedures, consider changing to a less nutritious substrate for initial spawning or sporeing into.

A good way to use highly nutritious spawn substrate, which is advantages for higher yields, is to buy presterilized compost/grain and just spawn into mycobags. Then when the mycobags are colonized, mix them in your fruiting substrate and you are pretty well protected if you have enough spawn, as mycelium will be faster at this point than other stuff most of the time.