The contents of $#{ ... } must be an expression that returns a reference or a variable name (aka symbolic reference).

$#{ @array } was buggy in the past — it was treated as $#{ \@array } — but it was fixed to work as documented. You were apparently relying on this bug. The correct usage is

$#{ $ref }
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or simply.

$#$ref
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Note the parallels:

$array[$i]  ${ $ref }[$i]    (also $ref->[$i])
@array      @{ $ref }
$#array     $#{ $ref }
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Ref:
Dereferencing Syntax
References Quick Reference

But when I ran the code that does this in perl 5.10.1, it returns -1 for $#{@{$a}}.

You should have gotten

Can't use string ("0") as an ARRAY ref while "strict refs" in use.
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Don't hide errors. Use «use strict; use warnings;»!