How can I expand variables in text strings?

Let's assume that you have a string like: 

    $text = 'this has a $foo in it and a $bar';
    $text =~ s/\$(\w+)/${$1}/g;

Before version 5 of perl, this had to be done with a double-eval substitution: 

    $text =~ s/\$(\w+)/$$1/eeg;

Which is bizarre enough that you'll actually need probably need an EEG afterwards. :-)