かもしれない with no extra element can represent any degree of uncertainty above 0% and below 100%. ひょっとしたら, literally like "if it were to happen", specifically implies a low probability. It is used with a connotation that the speaker did not recognize the possibility or evaluate it likely beforehand, or it is an unwelcome assumption, etc.
By the way, ひょっと is practically only used in three derivative forms: ひょっとしたら, ひょっとすると, and ひょっとして. While they are all possibly translated into "perhaps", ひょっとしたら grammatically leans most toward conditional clause and ひょっとして to adverb, which results in uninterchangeability in some situations.
Are you perhaps Keanu Reeves?