#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use Lingua::EN::Numbers qw{ num2en };

my $text = 'abc No. 347 xyz';

$text =~ s/No\. \K(\d+)/join "", "\\", map num2en($_), split m{}, $1/ge;

print $text;  # abc No. \threefourseven xyz