I'm not aware of any pact being signed in the 1980s with Iran. However, I assume maybe Zia-ul-Hq renewed the agreement with Iran because there was a massive governmental change.
I am also aware that Saddam Hussain was smuggling weapons to the Balochistan's Bugti separatists in the 1980s because Pakistan opted to stay neautral in the Ian-Iraq war. As a result Iran informed Pakistan of what was going in the consulates/Emabassies, which lead to the arrest of Iraqi official/s (ambassador?). Maybe some sort of pact was signed between Iran and Pakistan then as a result of that development, but I cannot confirm it.
Serious accusations my friend.
p.s. Zia did sign a pact, I cited my reference above. I am not blaming Zia, btw.