Any ideas how I can make this better?

Any will do it! Specifically, the any function from the List::Util module:

use List::Util qw( any );

    next  if any { $_ eq $offer{retailer} }  @ignore_merchants;
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Many cases of using grep in a conditional can be written using any instead, as it can short-circuit after the first true result.

Of course, a hash is better if you're going to be doing this check a lot of times.