Another option: convert the data to a JSON string base64-encoded JSON string, then sign the string.

use JSON::XS;
use Digest::MD5 'md5_hex';
use MIME::Base64;

my $signing_key = '//Sw0rdFiSh//';
my $data = {
  first_name => 'Will',
  last_name  => 'Smith',
  age        => 39,
};
my $json_string    = encode_json( $data );
my $signature      = md5_hex( $signing_key . $json_string );
my $base64_encoded = encode_base64( $json_string, '' );

# Send the encoded data and the signature in the HTML as hidden fields
print <<"HTML";
<input type="hidden" name="_signature" value="@{[ $signature ]}" />
<input type="hidden" name="_data" value="@{[ $base64_encoded ]}" />
HTML
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Later, when processing the stored data on another step:

my $json_string   = decode_base64( $q->param('_data') );
my $new_signature = md5_hex( $signing_key . $json_string );
unless( $new_signature eq $q->param('_signature') )
{
  die "The data has been tampered with";
}# end unless()
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