my %replaces = (
  email => sub { "<a href=mailto:'$_[0]'>$_[1]</a>" },
  url => sub { "<a href='$_[0]'>$_[1]</a>'" },
  # etc etc etc 
);
sub replace {
  my ($tag,$attr,$content) = @_;
  $attr ||= $content;
  return $replaces{$tag}->($attr,$content);
}