A string can become a number again :) Its the flags that reveal the true nature (last touch)

use JSON qw/ to_json from_json /;
use Devel::Peek qw/ Dump /;
my $q = 3;
print "$q\n";
Dump( $q );
$q++;
Dump( $q );
print to_json( { a => 3, qw/ b 3 q/, $q } );
__END__
3
SV = PVIV(0x9a3778) at 0x99b524
  REFCNT = 1
  FLAGS = (PADMY,IOK,POK,pIOK,pPOK)
  IV = 3
  PV = 0xad23fc "3"\0
  CUR = 1
  LEN = 12
SV = PVIV(0x9a3778) at 0x99b524
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 4
  PV = 0xad23fc "3"\0
  CUR = 1
  LEN = 12
{"a":3,"q":4,"b":"3"}

http://search.cpan.org/dist/illguts/index.html