$foo{$a,$b,$c}

it really means

    $foo{join($;, $a, $b, $c)}

But don't put

    @foo{$a,$b,$c}      # a slice--note the @

which means

    ($foo{$a},$foo{$b},$foo{$c})

Default is ``\034'', the same as SUBSEP in awk. Note that if your keys contain binary data there might not be any safe value for ``$;''. (Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon. Yeah, I know, it's pretty lame, but `` $,'' is already taken for something more important.)

Consider using ``real'' multidimensional arrays.