"Having come in the flesh" represents a fairly literal rendering of the perfect participle in 1 John 4:2.
I notice that J.B. Rotherham also translates the verse:
"Hereby do ye perceive the Spirit of God:--
W. Hall Harris points out that the Greek expression ὁμολογεῖ Ἰησοῦν Χριστὸν ἐν σαρκὶ ἐληλυθότα is literally "confess Jesus Christ in the flesh having come."
On perfect participles in general, Daniel B. Wallace makes this observation:
"The perfect participle is almost always antecedent with reference to the main verb. When it is contemporaneous, such is due to either an intensive use of the perfect or to a present force of the perfect in its lexical nuance" (Greek Grammar Beyond the Basics, page 626).
But notice that John replaces the perfect participle with the present form of the Greek verbal adjective in 2 John 7 (ἐρχόμενον). This syntactical structure makes the translational issue a little more complex; however, the perfect participle no doubt refers to antecedent action in the first Johannine passage (1 John 4:2) whereas the present participle likely has the same reference in 2 John 7.