Is this the "infinitive" form if the verb or is it conjugated to be an imperative? Does bahasa indonesia even have verb conjugations?
Does it perhaps also mean something like "get lost"?
Pergilah ... -lah adds emphasis and would be better if you wanted to express 'go away' to thar extent.
-lah can also be used to often a command or add politeness to it ... depending upon the tone (add or don't add the exclamation mark).
Leave or get lost