Note that this effect isn't unique to Data::Dumper. Any operation that stringifies a number or integer will change how JSON (the pure Perl implementation, I didn't look at the XS) represents it.

use strict;
use warnings;
use JSON;

my $json = JSON->new;
my $data = { foo => 'bar', qux => 0.42 };

printf "before: '%s'\n", $json->encode($data);
print "$data->{qux}\n";
printf "after: '%s'\n", $json->encode($data);
[download]

gives

before: '{"qux":0.42,"foo":"bar"}'
0.42
after: '{"qux":"0.42","foo":"bar"}'
[download]

This problem isn't unique to JSON either. I have seen similar issues in Win32::OLE, for example. Anything that inspects the scalar type and behaves differently depending on what it finds must resolve the ambiguity, shown by ikegami / Devel::Peek, that sometimes exists. When using such modules, you must be careful with your data.

One way to avoid the problem would be to make a copy of your data and stringify the copy.