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$data->{foo} = "";

if ( exists $data->{foo} ) {
    print $data->{foo}{bar};    # 'Can't use string ("") as a HASH ref
+...'
}

$data->{foo} = undef;

if ( exists $data->{foo} ) {
    print $data->{foo}{bar};    # 'Use of uninitialized value in print
+...'
}