How can I expand variables in text strings?
    Let's assume that you have a string that contains placeholder variables.

        $text = 'this has a $foo in it and a $bar';

    You can use a substitution with a double evaluation. The first /e turns
    $1 into $foo, and the second /e turns $foo into its value. You may want
    to wrap this in an "eval": if you try to get the value of an undeclared
    variable while running under "use strict", you get a fatal error.

        eval { $text =~ s/(\$\w+)/$1/eeg };
        die if $@;

    It's probably better in the general case to treat those variables as
    entries in some special hash. For example:

        %user_defs = (
            foo  => 23,
            bar  => 19,
        );
        $text =~ s/\$(\w+)/$user_defs{$1}/g;