This blog post discusses the best answers to this question.

As a short summary, if you can install CPAN modules then the best solutions are:

if any(@ingredients) eq 'flour';
[download]

or

if @ingredients->contains('flour');
[download]

However, a more usual idiom is:

if any { $_ eq 'flour' } @ingredients
[download]

which i find less clear.

But please don't use the first() function! It doesn't express the intent of your code at all. Don't use the "Smart match" operator: it is broken. And don't use grep() nor the solution with a hash: they iterate through the whole list. While any() will stop as soon as it finds your value.
Check out the blog post for more details.

PS: i'm answering for people who will have the same question in the future.