Perceive="feel, or feel more acutely". People with HA tend to notice more bodily sympotms than those wihtout HA. In addition, it is very common to notice more symtpoms on one side over the other. It does not mean the symtpom is imagined--but it does mean you are amplifying more on one side (since the brain perceives all input, without the brain interpreting and potentially amplifying the senseation, you would not feel anything even if your leg were on fire).
I hope it helps reassure you that it is very common to become concerned about the symtpoms on a single side--and that the symtpoms to not need to be "evenly distributed" for it to be health-anxiety related.