How exactly does copper react with hydrogen peroxide, salt and citric acid?
The acids, hydrochloric or citric, have a common property : they produce $\ce{H+}$ in water. These ions are necessary to react with copper and hydrogen peroxide $\ce{H2O2}$. Indeed metallic copper reacts with hydrogen peroxide $\ce{H2O2}$ and an acid in the following way : $$\ce{Cu + H2O2 + 2 H+ -&gt; Cu^{2+} + 2 H2O}$$ So whatever the acid chosen, hydrochloric or citric, the copper will be dissolved the same way. With citric acid, the dissolution should be slower. But it leads to the same final result : metallic copper will be destroyed. The only difference is the final composition of the liquid. With HCl, the final liquid is made of $\ce{Cu^{2+}}$ and $\ce{Cl-}$ ions, which may be partially recombined. With citric acid, the final liquid is made of citrate ions, which will make a complex with $\ce{Cu^{2+}}$. Citral ions from citric acid will make a small change of color, but has no other effect. Both solutions are equally dangerous, or equally non-dangerous if you prefer, because copper solutions are always toxic. You should take care of the obtained copper solutions in the same way, whatever their compositions.
Personally I would recommend using citric acid instead of hydrochloric acid. It is less corrosive, and has nearly the same effect on copper + $\ce{H2O2}$.