Sorry, but I can't explain why.

Because the .* before "vodka" eats "vodka" as well, as you can easily see if you add () around .*:

my $re = qr/beer=(\d{2})(.*)(?:vodka=(\d{2}))?(.*)chips=(\d{3})/;
my $str1 = "beer=10&otherstuff&vodka=20&otherstuff&chips=100";
my $str2 = "beer=10&otherstuff&juice=20&otherstuff&chips=100";
say for $str1 =~ /$re/g;
say "==";
say for $str2 =~ /$re/g;
[download]

Changing the first .* to .*? does not help, either, because then the second .* eats "vodka".