use strict;
my $str = ' <list name="ItemList">
<item>
<p value="id">1445</p>
<p value="IP">10.25.30.19</p>
<p value="Prflen">26</p>
</item>
<list name="ItemList">
<item>
<p value="id">1499</p>
<p value="IP">10.40.50.53</p>
<p value="Prflen">26</p>
</item>
<list name="ItemList">
<item>
<p value="id">1445</p>
<p value="IP">10.45.70.19</p>
<p value="Prflen">26</p>
</item>
<list name="ItemList">
<item>
<p value="id">1446</p>
<p value="IP">10.77.90.55</p>
<p value="Prflen">26</p>
</item>';
my $regex = qr/(<p value="id">(?=1445).+\R.+<p value="IP">)(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})(.+)/mp;
my $subst = '\\110.25.30.19\\3';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html