Let me guess... LPC? :-) There have been a few questions in that direction lately. I had not heard of it before.

Perhaps you should use an anonymous array for a "key", instead of a list. You then no longer need to wrap each pair in an anonymous array. The resulting code would look more Perlish, IMO.

my %hash = expand_keys(
    ['key1', 'key2', 'key3'] =>
      'Value for key1, key2, and key3',
    [qw( key4 key5 key6) ] =>
      'Value for key4, key5, key6',
    'simple key' => 'Value for simple key'
);
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Now it's just a matter of defining expand_keys():

sub expand_keys {
    my @result;
    while(@_) {
        my $key = shift;
        my $value = shift;
        if(ref $key) {
            push @result, $_, $value foreach @$key;
        } else {
            push @result, $key, $value;
        }
    }
     return @result;
}
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You could populate a hash instead of pushing the data onto an array, in the sub, I suppose. I think it might be a tiny bit slower, because you would then be building two hashes, instead of just one (and an array).