What version of JSON module? I'm on Windows 10 64-bit with Strawberry 5.24.1 MSWin32-x64-multi-thread. It seems your desired output works for me.

Perl> use JSON;
Perl>
Perl> my $json =
More?     JSON->new->allow_nonref->allow_unknown->allow_blessed->prett
+y(1);
Perl> my $data3;
Perl> $data3->{"num"} = "3";
Perl> my $addr = $data3->{"num"} + 7;
Perl> my $body3 = $json->encode($data3);
Perl> print $body3;
{
   "num" : "3"
}
Perl> print $JSON::VERSION;
2.90
Perl> exit
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