The ornamental design for a perambulator, as shown and described.
FIG. 1 is a front perspective view of the perambulator;
FIG. 2 is a rear perspective view of the perambulator;
FIG. 3 is a front elevational view of the perambulator;
FIG. 4 is a rear elevational view of the perambulator;
FIG. 5 is a right side elevational view of the perambulator; and,
FIG. 6 is a left side elevational view of the perambulator.
The crosshatching shown in the above Figures is understood to represent a mesh material. Features of the perambulator shown in phantom are shown for environmental purposes and do not form any part of the claimed design.