In Suetonius's Vita Horati, a letter from Augustus to Horace is quoted, which includes the sentence:
Sume tibi aliquid iuris apud me, tamquam si convictor mihi fueris.
The Loeb translation gives: "Enjoy any privilege at my house, as if you were making your home there".
Why the perfect subjunctive fueris?
Bennett's New Latin Grammar, section 307, discusses the forms of verbs after such phrases as tamquam si:
Conditional Clauses of Comparison are introduced by the particles, ac sī, ut sī, quasi, quam sī, tamquam sī, velut sī, or simply by velut or tamquam. They stand in the Subjunctive mood and regularly involve an ellipsis (see § 374, 1), as indicated in the following examples:—
tantus patrēs metus cēpit, velat sī jam ad portās hostis esset, as great fear seized the senators as (would have seized them) if the enemy were already at the gates; sed quid ego hīs testibus ūtor quasi rēs dubia aut obscūra sit, but why do I use these witnesses, as (I should do) if the matter were doubtful or obscure; serviam tibi tam quasi ēmerīs mē argentō, I will serve you as though you had bought me for money.
Note that in sentences of this kind the Latin observes the regular principles for the Sequence of Tenses. Thus after principal tenses the Latin uses the Present and Perfect (as in the second and third examples), where the English uses the Past and Past Perfect.
Now, in this example we're in primary sequence. So if the usual sequence of tenses rules are in force, the perfect subjunctive should express prior time: "as if you had (at some time in the past) been living with me". This seems not to be what's intended, though – rather, it looks like a present counterfactual: "as if you were (now) living with me". But in that case, shouldn't we expect either sis (if following sequence of tenses), or maybe esses (as in an ordinary present counterfactual condition)?
As a bonus question, how does the Loeb get to "enjoy any privilege at my house"? AFAIK, aliquis does not mean "any whatsoever" (that's quicumque or quilibet), but "some", so it should literally be "take some (not further identified) kind of privilege at my house".