Actually, you were quite close to it, what you could use here is a Hash of Hashes to hold the data:

my $rhChildren = {
   {'Peter'} => {
      child_name => 'Peter',
      teacher_name => 'Ms. Jones',
      room_number => '503'
   }
};   #add records as needed

my $text = "Your child #child_name# is invited to come to our school. 
+Your child's teacher will be #teacher_name#, and the class is held in
+ room #room_number.";

foreach my $child (keys %$rhChildren) {
   my $substText;
   ($substText = $text) =~ s/\#([^#]+)\#/$rhChildren->{$child}{$1}/g;
   print "$substText\n";
}
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OK, so there is a redundancy in using the child's name both as key and value in the hash, but it works and uses regexes, right?