You could use a CPAN module, such as
XML::Twig. The following code unconditionally replaces the text of all instances of 'count' elements with the value 14.
use strict;
use warnings;
use XML::Twig;
my $xmlStr = <<XML;
<project>
<run_count>
<count>13</count>
</run_count>
</project>
XML
my $twig= new XML::Twig(
PrettyPrint => 'indented',
twig_handlers => { count => \&count }
);
$twig->parse($xmlStr);
$twig->print();
exit;
sub count {
my ($twig, $cnt) = @_;
$cnt->set_text(14);
}
__END__
<project>
<run_count>
<count>14</count>
</run_count>
</project>
Side note: Welcome to the Monastery. In general, it is better to use 'code' tags instead of 'pre' tags when posting code. (see Perl Monks Approved HTML tags)