Rather than iterating through %whop, you can use reverse to swap the keys and values of a hash, then use exists:

my %whop_value = reverse %whop;
foreach my $newm (@newest_members) {
    $newm .= '<b>*</b>' if exists $whop_value{$newm};
    $newm .= ' - ';
    print $newm;
}
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Using reverse on hashes is described in perlfunc:reverse