To save myself nightmares I always worked out of a hash and skipped the extra eval and the dollarsigns... Others already noted your match issue. Me, I was even more careful and set rules on myself. I'm a pretty shifty fellow and I have to keep a close eye on me to keep from starting trouble. =)

#!/usr/bin/perl -w
use strict;

my %h = ( name=>"Mark", monk=>"extremely", typo=>"Weaknesses");

while (<DATA>) {
  s/<~([a-z][a-z0-9]*)~>/$h{$1}/g;
  print;
}
__DATA__
[<~monk~>|<~name~>] also wrote it like this:
  s/<~([a-z][a-z0-9]*)~>/exists $h{$1} ? $h{$1} : "<~$1~>"/eg;
to expose <~typos-> and survive "-w" and "use strict".
Try both lines!
[download]

--
$you = new YOU;
honk() if $you->love(perl)