I've continued to search for documentation about this pecularity to no avail

From perldata:

Within search patterns (which also undergo double-quotish substitution) there is an unfortunate ambiguity: Is /$foo[bar]/ to be interpreted as /${foo}[bar]/ (where [bar] is a character class for the regular expres- sion) or as /${foo[bar]}/ (where [bar] is the subscript to array @foo)? If @foo doesn't otherwise exist, then it's obviously a character class. If @foo exists, Perl takes a good guess about [bar], and is almost always right. If it does guess wrong, or if you're just plain paranoid, you can force the correct interpretation with curly braces as above.