When one stringifies a scalar, the string is attached to the scalar as an optimisation. It doesn't remove the existing number. This isn't a bug in D::D, but it doesn't mean that D::D couldn't be changed to avoid adding a string to the scalar. It already does in other circumstances.

(pPOK = has string)

>perl -MDevel::Peek -MData::Dumper -e"$_=0.42; Dumper($_); Dump($_);"
SV = NV(0x3335fc) at 0x328fe4
  REFCNT = 1
  FLAGS = (NOK,pNOK)
  NV = 0.42

>perl -MDevel::Peek -MData::Dumper -e"$_={ qux => 42 }; Dumper($_); Du
+mp($_->{qux});"
SV = IV(0x57b260) at 0x57b264
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 42

>perl -MDevel::Peek -MData::Dumper -e"$_={ qux => 0.42 }; Dumper($_); 
+Dump($_->{qux});"
SV = PVNV(0x5490e4) at 0x25b264
  REFCNT = 1
  FLAGS = (NOK,POK,pNOK,pPOK)
  IV = 0
  NV = 0.42
  PV = 0x4c9f84 "0.42"\0
  CUR = 4
  LEN = 36
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The problem is that JSON doesn't allow you to specify how to handle a variable that's both a float and a string.