gistfile1.xml

<flow name="groovyTransformerWithParameters">
        <script:transformer name="stringReplaceWithParams">
                <script:script engine="groovy">
                    <property key="oldStr" value="l" />
                    <property key="newStr" value="x" />
                    <script:text>
                        return payload.toString().replaceAll("$oldStr", "$newStr")
                     </script:text>
                </script:script>
        </script:transformer>
</flow>