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';
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if @ingredients->contains('flour');
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if any { $_ eq 'flour' } @ingredients
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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.