A hash is a data-structure, like an array. The difference is that the lookup is not done via a numerical index, but with a key. Eg., if you create a hash like this:

%hash = ( "keys" => "value" );
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You will be able to refer to "value" by saying:

print $hash{"value"};
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In your case

print $wanted{"members.aol.com"};
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will print "1". So if your regex matches, the matched part is used to make a lookup in your hash. If that returns true, the $line is printed.

You should definitly read perldata

Note: I am a bit surprised that you seem to understand a regex, but have problems with something very basic like hashes.