The expression in the curlies should evaluate to a reference (or variable name when strictures are off). You were relying on a case where this was broken. Looks like the bug was fixed in 5.10.0:

>perl589\bin\perl -le"@8 = 0..5; @a = 0..7; print $#{ @a }"
7

>perl5100\bin\perl -le"@8 = 0..5; @a = 0..7; print $#{ @a }"
5
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See Dereferencing Syntax and References Quick Reference

was coming back as -1.

Only because you disabled strictures. (boooo!)

>perl -wle"no strict;  my @a = qw( a b c ); print $#{ @a }"
-1

>perl -wle"use strict; my @a = qw( a b c ); print $#{ @a }"
Can't use string ("3") as an ARRAY ref while "strict refs" in use at -
+e line 1.
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Update: Added examples.