How to blend images in PerlMagick

I can't figure out how the Composite Blend is used in PerlMagick?  For example,
the command line way to average two images is:

  composite -blend 50%


So, how would this command be written in PerlMagick?  If I write the following
command, I get an internal server error. :-(

  $photo->Composite(image => $imagevariable,compose => 'Blend');


Also, is there a good spot on the web to see examples of how PerlMagick
commands are actually used?