What I am missing here?

When they're in a file that isn't a perl program, and perl isn't executing that file, perl doesn't consider them perl code, so there is no interpolation that perl will do

What you've got there is called a template, so you need a template engine to do interpolation, like String::Interpolate/ String::Interpolate::RE , String::Defer, Interpolation, Template,Text::Xslate...