No, you have it backwards. The hash equivalent of
@$ar[0] (or @$ar[0, 1, 2])
is
@$hr{a} (or @$hr{'a', 'b', 'c'})

%$hr{a} is nonsense. Read about slices in perldata

By the way, you shouldn't use slices for single elements.
@$ar[0] and @$hr{a}
should be
$$ar[0] and $$hr{a}

Remember: When using an index on a hash or an array, the sigil is of the type returned/assigned. Slices return/accept a list, so @ is used. When refering to a single element, $ is used. % is never used when an index is specified.

Update: Here's a table which should help:

		Direct	Using References
			Syntax 1*	Syntax 2
array	element	$a[0]	${$ar}[0]	$ar->[0]
	slice	@a[0,1,2]	@{$ar}[0,1,2]
	unindexed	@a	@{$ar}
hash	element	$h{'a'}	${$hr}{'a'}	$hr->{'a'}
	slice	@h{'a','b','c'}	@{$hr}{'a','b','c'}
	unindexed	%h	%{$hr}

* – The curly brackets around $ar and $hr are optional.

Very nice table/overview!!!