Substitute values from property file in Jenkinsfile for dynamically set variables - groovy

Question

I have a Jenkinsfile with multiple parameters that are environment-specific.
The values for those parameters are stored in a property file. I am trying to set the variables based upon the selected environment and then replace their values from the property files.
Jenkinsfile
ENVIRONMENT is a choice parameter having 2 values: ft, perf and pm as below in my Jenkinsfile
choice (
name: 'ENVIRONMENT',
choices: ['ft', 'perf','pm'],
description: 'please select the environment'
)
PROPERTY_FILE is another choice parameter as below
choice (
name: 'PROPERTY_FILE',
choices: ['jenkins/app.groovy'],
description: 'please select the property file'
)
app.groovy looks like:
Test_perf="Hello"
Test_ft="World"
Test_pm="Welcome"
Stages
stage('Load Environment Property File') {
steps {
script {
//sourcing user selected property file
load "${PROPERTY_FILE}"
def envName = "${PROPERTY_FILE}".tokenize(".")[0]
//it will set build description as description in Jenkins build history
env.envName=envName
currentBuild.description = "Env:${envName}"
}
}
}
//STAGE FOR WHICH I NEED HELP
stage('Set Variables Based upon environment name') {
steps {
script{
if ("${ENVIRONMENT}" == "perf" ){
var="\${Test_"+"${DEPLOYMENT_ENVIRONMENT}"+"}"
echo "${var}" //output is ${Test_pm} and is correct as the substitution happens.
//NOW, I AM TRYING TO echo "HELLO", i.e.; replacing the variable "${Test_perf}" stuffed inside "var" from the property file. However, since the property file is already loaded, it will not replace the key by its value from the property file.
} else if ("${ENVIRONMENT}" == "ft" ){
var="\${Test_"+"${DEPLOYMENT_ENVIRONMENT}"+"}"
echo "${var}" //output is ${Test_ft} and is correct as the substitution happens.
//NOW, I AM TRYING TO echo "World"
} else {
var="\${Test_"+"${DEPLOYMENT_ENVIRONMENT}"+"}"
echo "${var}" //output is ${Test_pm} and is correct as the substitution happens.
//NOW, I AM TRYING TO echo "Welcome"
}
}
}
}
Is there a way to further substitute the value from the property file. For e.g.; "${var}" expands to "${Test_pm}" and then "${Test_pm}" gets the value "Welcome" from the property file.
Please help.