Like many Domesday measures, there is no certainty about the extent of the customary perch, or even whether it was a linear or areal unit, though it is most commonly used in Domesday as one of the dimensions of an area.
As a measure of length, it would have been notionally 5.5 yards, perhaps in origin the width of 4 oxen yoked side-by-side. As a measure of area, the perch was a virgate, a quarter of an acre. Customary furlongs of 40 perches are recorded in the next century.
For more detail, see F.W. Maitland, Domesday Book and beyond (1897); Philip Grierson, English linear measures: an essay in origins (1972); and Philip Grierson, 'Weights and measures', Domesday Book: studies, edited by Ann Williams and R.W.H. Erskine (1987), pages 80-85.