my $k1a = "a$;b";
my $k1b = 'c';
my $k2a = 'a';
my $k2b = "b$;c";

$multi{$k1a, $k1b} = 1;
$multi{$k2a, $k2b} = 2;

print $multi{$k1a, $k1b};  # 2!!