I did not ask you to define the hash/collection. I wanted you to show us how does the hashLookup() function look like. If you want a "complete" example so let's define a function that receives a template and a hash containing the values to insert into the template and returns the text with the values:

sub fillTemplate {
  my ($text, $hash) = @_;
  $text =~ s/%(\w)%/$hash->{$1}/g;
  return $text;
}
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How complex and twisted is the code in C#? Especialy considering the fact that the second parameter for Regex.Replace() is a delegate and that C# doesn't support unnames functions and closures? You have to define the hashLookup() as a fullblown method and you have to find a way to pass it the collection somewhere behind the scenes.

Either it can be a static method and then you'll have a static field in the class that'll hold the reference to the collection. Or you can create a brand new class that'll contain just the hashLookup() method and the reference to the collection. Or do I miss anything?

This is not the difference between Pascals begin-end and C's {}, there I would agree it's just an unimportant matter of verbosity. This is something so awkward, lengthy and unmaintainable that noone can be expected to use regexps. Except maybe for validations.