The __DATA__ method seems to definitely be the right way here (w/purely static string data), but it in the interest of plugging a module i find handy (and preparing for more complicated scenarios, keep in mind something like this should work as well, using IO::Scalar:

my $data = <<EOF;
<?xml version='1.0' standalone='yes'?>
<RULE output_format="pdf" lsog_version="lsog5">
  <SERVICE_REQUEST>
    <SOMETAG min_length="25" max_length="60">Some value</SOMETAG>
    <ANOTHER_TAG min_length="0" max_length="8">another value</ANOTHER_
+TAG>
  </SERVICE_REQUEST>
</RULE>
EOF
use IO::Scalar;
my $SH = new IO::Scalar \$data;
my $template = $xs->XMLin( $SH );
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