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

my $in = <<EOT;
         <Location />
             MaxConnPerIP 12
             NoIPLimit image/*
         </Location>
EOT
my $max = 16;

my $out = $in;
$out =~ s/(<Location .+?MaxConnPerIP\s+)(?:\d+)(.+?<\/Location>)/$1$max$2/s;
print $out;