It may be a bit of an overkill, but assuming you are trying to have:

$text = 'my $foo has gone completely $bar';
%hash = (foo => 'dog',  bar => 'crazy');
$final_text = unknown_method($text);
#$final_text is now 'my dog has gone completely crazy'
[download]

I think Text::Template will do exactly what you want. If that's not what you want, please clarify.