From JSON::XS (emphasis added)

$json = $json->allow_nonref ([$enable])
$enabled = $json->get_allow_nonref
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If $enable is true (or missing), then the encode method can convert a non-reference into its corresponding string, number or null JSON value, which is an extension to RFC4627. likewise, decode will accept those JSON values instead of croaking.

If $enable is false, then the encode method will croak if it isn't passed an arrayref or hashref, as JSON texts must either be an object or array . Likewise, decode will croak if given something that is not a JSON object or array.

Example, encode a Perl scalar as JSON value with enabled allow_nonref, resulting in an invalid JSON text:

       JSON::XS->new->allow_nonref->encode ("Hello, World!")
       => "Hello, World!"
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In short, according to RFC4627 a JSON-string must start with an array (i.e. [...] ) or hash (i.e. {...} ) on top level.

Since many applications accept more, you have the option allow_nonref to allow simple values and strings.

As a side note: How do I post a question effectively?