Step 1: Use the perchlorate structure:
In perchlorate, chlorine is bonded to four oxygens, each pulling electrons towards itself. Chlorine uses all 7 valence electrons.
Step 2: Count electron shifts:
Four oxygens take $4 \times 2 = 8$ electrons in total (as $-2$ each). The ion has only one extra electron, so Cl must have given $8 - 1 = 7$ electrons.
Step 3: Result:
Cl has lost 7 electrons, so its oxidation number is +7. Option B is correct.
Final Answer:
Chlorine has lost 7 electrons, giving an oxidation number of +7.
\[ \boxed{\text{(B) }+7} \]