my $tag = 'conditionning';
my $newval = '200';

my $data = join('', <DATA>);
$data =~ s/(<$tag[^>]*>)[^<]*(<\/$tag[^>]*>)/$1$newval$2/gi;
print $data;

__DATA__
<category name="Coffee">
    <heading>150 years of know how at the service of a same strategy: Taste</heading>
  <photo>images/bag_legal.jpg</photo>
  <subcategory name="Prestige">
     <photo>images/prestige.jpg</photo>
    <product>Coffee
                <brand>33</brand>
      <channel>D</channel>
      <channel>A</channel>
      <conditionning unit="gr">250</conditionning>
      <description>aaaa</description>
      <packaging_qty>12</packaging_qty>
      <product_id>8080</product_id>
      <recipe>/recipes/coffee1.html</recipe>
    </product>
</subcategory>
</category>