my $text = << 'end_of_test';
Your child <child> is invited to
come to our school.  Your child's
teacher will be <teacher>, and the
class is held in <room>.";
end_of_test

my %hash = ('child' => 'mary',
            'teacher' => 'mrs b',
            'room' => '123');

print "Raw: $test\n";

$test =~
  s[<(\w+)>]
   [defined($hash{$1}) ? $hash{$1} : warn]ieg;

print "Processed: $test\n";