Here's one way:

$string =~ s/description/summary/gi ;
$string =~ tr/=/:/ ;
[download]

and here's another:

%subs = (
  description => 'Summary',
  '=' => ':',
  blah => 'wibble'
);
foreach (keys %subs) {
  $string =~ s/\Q$_/$subs{$_}/gi;
}
[download]

hth,
andy.