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>