there may be a better way to scan that hash for an existing user entry in the DB

Indeed - instead of storing your identified usernames in an array (list), create a hash of existing user names. Then you can do:

if ($existing_users{$key} ) {
...do some stuff...
}
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Where, instead of pushing the user names retrieved from your db onto a @existing_users array, you would have populated a %existing_users hash with 1's using the user id's as the keys