Extraction of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>V</mml:mi></mml:mrow><mml:mrow><mml:mi>ub</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>from the Decay<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="italic">B</mml:mi><mml:mi/><mml:mo>→</mml:mo><mml:mi>π</mml:mi><mml:mi mathvariant="italic">l</mml:mi><mml:mi>ν</mml:mi></mml:math>

We develop the perturbative QCD formalism for semileptonic $B$ meson decays, which includes Sudakov suppression on the spatial extent of a heavy meson containing a light valence quark. We show that the perturbative calculation for the spectrum of the decay $B\ensuremath{\rightarrow}\ensuremath{\pi}l\ensuremath{\nu}$ is reliable for the energy fraction of the pion above 0.3. Combining predictions from soft pion theorems, we obtain an upper limit of the matrix element $|{V}_{\mathrm{ub}}|$ of roughly $3.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$.