Is it possible to check a variable against all the keys in a hash

The question suggests you do not understand the fundamental property of a hash.

The primary use of a hash is for a quick and simple lookup of a 'key', by indexing, using curly-braces.

The first line of your code:

if (! exists ($hash{$remoteIP})){
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does exactly that - looks up the value of $remoteIP, and can be thought of as comparing with every possible key in %hash, and returning 'true' if the matching one exists.

Your later comparison:

    if ($remoteIP ~~ @k){
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is a more expensive way of doing the same comparison that the first line does.