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%seen = (
    paul  => 1,
    jane  => 1,
    betty => 1,
);

print for keys %seen;
# will print paul, jane, betty (though not necessarily in this order, 
+see note below)

$seen{paul} = 1;       # this does absolutely nothing! but...
++$seen{paul};         # paul's *value* is 2 now
print for keys %seen;  # yet this still prints the same list

@list = grep !$temp{$_}++, @list; # same thing, shorter.