How can JSON for perl distinguish between “string” and “numeric” ?

JSON.pm distinguish between “string” and “numeric” , and convert from perl object to json string.

#!/usr/local/bin/perl
use strict;
use JSON;

main();
sub main {
    print JSON::encode_json({val=>'10'}),"\n";
    print JSON::encode_json({val=>10}),"\n";
}
1;

↑↓

$ ./test_encode_json.pl 
{"val":"10"}
{"val":10}

Document of JSON describe as belog,

number
A JSON number becomes either an integer, numeric (floating point) or string scalar in perl, depending on its range and any fractional parts. On the Perl level, there is no difference between those as Perl handles all the conversion details, but an integer may take slightly less memory and might represent more values exactly than floating point numbers.

If the number consists of digits only, JSON will try to represent it as an integer value. If that fails, it will try to represent it as a numeric (floating point) value if that is possible without loss of precision. Otherwise it will preserve the number as a string value (in which case you lose roundtripping ability, as the JSON number will be re-encoded to a JSON string).

JSON::PP backend of JSON uses B module

There is source code snippet of JSON::PP.

sub value_to_json {
    my ($self, $value) = @_;

    return 'null' if(!defined $value);

    my $b_obj = B::svref_2object(\$value);  # for round trip problem
    my $flags = $b_obj->FLAGS;
    ######## HERE !!!! ########
    return $value # as is 
        if $flags & ( B::SVp_IOK | B::SVp_NOK ) and !( $flags & B::SVp_POK ); # SvTYPE is IV or NV?

    my $type = ref($value);

    if(!$type){
        return string_to_json($self, $value);
    }
    elsif( blessed($value) and  $value->isa('JSON::PP::Boolean') ){
        return $$value == 1 ? 'true' : 'false';
    }
    elsif ($type) {
        if ((overload::StrVal($value) =~ /=(\w+)/)[0]) {
            return $self->value_to_json("$value");
        }

        if ($type eq 'SCALAR' and defined $$value) {
            return   $$value eq '1' ? 'true'
                   : $$value eq '0' ? 'false'
                   : $self->{PROPS}->[ P_ALLOW_UNKNOWN ] ? 'null'
                   : encode_error("cannot encode reference to scalar");
        }

         if ( $self->{PROPS}->[ P_ALLOW_UNKNOWN ] ) {
             return 'null';
         }
         else {
             if ( $type eq 'SCALAR' or $type eq 'REF' ) {
                encode_error("cannot encode reference to scalar");
             }
             else {
                encode_error("encountered $value, but JSON can only represent references to arrays or hashes");
             }
         }

    }
    else {
        return $self->{fallback}->($value)
             if ($self->{fallback} and ref($self->{fallback}) eq 'CODE');
        return 'null';
    }

}

http://search.cpan.org/perldoc?JSON%3A%3APP

You can distinguish between "string" and "numeric" too, as below.

#!/usr/local/bin/perl
use strict;
use B ();

main();
sub main {

    for my $value (10, "10", 10.0,  10.1, "１０"){
        my $b_obj = B::svref_2object(\$value);
        my $flags = $b_obj->FLAGS;
        my $comp_1 = ($flags & ( B::SVp_IOK | B::SVp_NOK )),"\n";
        my $comp_2 =  ( $flags & B::SVp_POK ),"\n";
        if( $comp_1 and not $comp_2){
            print "$value is NUMERIC\n";
        } else {
            print "$value is            STRING\n";
        }
    }
}