use strict;
use warnings;
use File::Slurp;

my $searchStr = "value=\".*\"";
my $suffix = "YEPPA";
my $replaceStr = "value=\"_" . $suffix . "\"";

my @lines = read_file( \*DATA );

foreach my $line(@lines){
    print "line --> $line";
    $line =~ s/$searchStr/$replaceStr/g;
    print "line --> $line";
}

__DATA__
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="ServiceId.suffix" value="_TEST" />
  </appSettings>
</configuration>

E:\develop>perl test.pl
line --> <?xml version="1.0" encoding="utf-8" ?>
line --> <?xml version="1.0" encoding="utf-8" ?>
line --> <configuration>
line --> <configuration>
line -->   <appSettings>
line -->   <appSettings>
line -->     <add key="ServiceId.suffix" value="_TEST" />
line -->     <add key="ServiceId.suffix" value="_YEPPA" />
line -->   </appSettings>
line -->   </appSettings>
line --> </configuration>
line --> </configuration>