Hi rsriram, try like this,

use strict;
use warnings;

my $str = '<tag1>Complete</tag1><tag2>3386</tag2><tag3>77844</tag3><ta
+g4>11</tag4><tag5>30</tag5><tag6>4.7</tag6>';

if($str =~ /<tag1>Complete<\/tag1>/i)
{
$str =~ s/<tag6>(.+?)<\/tag6>/"<tag6>".( $1 * 100 )."<\/tag6>"/ei;

}
print $str;
[download]

Updated

Typo error in your replacement statement. Use 'e' option modifier for execute the statement in the replacement part.

See the documentation for further clarification perlre