It's not exactly the best golf (since I'm not much of a golfer-- I prefer readable code that passes strict), but here's the basic idea in the most concise "idiomatic" Perl I can write. You throw a wrench into the works with the list that translates to 'foo => zoo moo too bar => car far bar => zar tar qux => mux'.

#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my $h = f(qw[ foo => zoo moo too
              bar => car far
              bar => zar tar
              qux => mux ]);

print Dumper $h;

sub f {
    my( %hash, $y );
    my $key = my $x = shift;
    while($y = shift){
        if( $y eq '=>' ){
            pop( @{$hash{$key}});
            $key= $x;
            $y = shift;
        }
        push( @{$hash{$key}}, $y );
        $x = $y;
    }
    return \%hash;
}
[download]

condensed I get to:

sub f
{
my(%h,$y);my$k=my$x=shift;while($y=shift){if($y eq'=>'){pop@{$h{$k}};$
+k=$x;$y=shift;}push(@{$h{$k}},$y);$x=$y;}return\%h;
}
[download]

for a score of 118.