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We report the measurement of over one hundred new rotational transitions of the internal rotor $HCOOCH3$ in its E symmetry state. The principal axis method has been used to analyze these and all previously collected data. For relatively high barriers. Such as that in methyl formate, the A and E states can be treated as distinct asymmetric tops: only the latter showing explicit perturbations due to the internal rotation. The splitting of the E levels has been calculated using successive Van Vleck transformations and two-by-two diagonalizations, simplified by the evaluation of the perturbation matrix elements in the symmetric top basis.