Update 78 Characters (has order wrong, 2 'push'es to 'unshift's for 6 more characters): (and see note)

sub f {
while(@_){while(($a=pop)ne'=>'){unshift@b,$a};unshift@{$h{pop()}},@b;@
+b=()}\%h
}
[download]

FSR, '=>' was not being picked up by the qw function in the version of perl here (5.005); I switched to a split on a q function to test the case given:

my @h = split /\s+/, 
         q[foo => zoo moo too bar => car far bar => zar tar qux => mux
+ ];

print Dumper f(@h);

# output is:
$VAR1 = {
          'foo' => [
                     'zoo',
                     'moo',
                     'too'
                   ],
          'bar' => [
                     'car',
                     'far',
                     'zar',
                     'tar'
                   ],
          'qux' => [
                     'mux'
                   ]
        };
[download]

-----------------------------------------------------
Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain