The low-fi way would be some cleverly crafted search-and-replace:

sub fill_template {
  my ($string,$values) = @_;
  $string =~ s!\$\{(.*?)\}!$values->{$1}!eg;
  $string
};
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But in most cases, you will want to simply use one of the various already existing templating systems, depending on if you only want to template HTML-like files or also other stuff:

• Template Toolkit, also at http://www.tt2.org, a general templating toolkit
• HTML::Template, a minimal templating system, geared for templating HTML
• Petal, a templating system geared for templating HTML which keeps the template arguments in HTML attributes so the template remains usable in WYSIWYG editors
• Various others