I can't reproduce that either. It still returns the value stored in the original JSON, as expected.

#!/usr/bin/perl
use strict;
use warnings;

use JSON::XS;

my $json = '{"feed":{"updated":{"$t":"2019-07-19T20:04:56.046Z"}}}';
my $decoded_json = decode_json($json);
print $decoded_json->{feed}->{updated}->{'$t'};  # 2019-07-19T20:04:56
+.046Z
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