In order to study wire array Z-pinch instabilities, perturbations have been seeded by etching 15μm diameter aluminum wires to introduce 20% modulations in radius with a controlled axial wavelength. These perturbations seed additional imploding structures that are studied experimentally on the 1MA, 250ns MAGPIE generator [S. V. Lebedev et al., Plasma Phys. Control. Fusion 47, A91 (2005)] and with three-dimensional magnetohydrodynamic calculations using the ALEGRA-HEDP [A. C. Robinson and C. J. Garasi, Comput. Phys. Commun. 164, 408 (2004)] and GORGON [J. P. Chittenden et al., Plasma Phys. Control. Fusion 46, B457 (2004)] codes. Simulations indicate that current path nonuniformity at discontinuities in the wire radius result in perturbation-induced magnetic bubble formation. Imploding bubbles originating from discontinuities are observed experimentally, and their collision on axis determines the start of the main x-ray pulse rise. These mechanisms likely govern dynamics of standard wire array Z pinches, and tailoring the profile of imploding mass may allow x-ray pulse shaping for inertial confinement fusion applications.