I realize your code was mainly for demonstration purposes, but it can be shortened considerably. I'm sure this can be golfed down even more, but I came up with the following snippet, which I feel is simple yet not too obfuscated (and also demonstrates the use of s///e, so I guess it's not really too OT):

#!/usr/bin/perl
use strict;
use warnings;

my $str = 'The %NOUN% can jump very %ADVERB% with his %APPENDAGES%';

my %wb = (
    NOUN   => "kangaroo",
    ADVERB => "far",
);

$str =~ s/%([^%]+)%/$wb{$1} || "\%$1\%"/ge;

print "$str\n";
[download]

Update: %s/\$template/\$str/g; silly mistake caused by last-minute-edits-without-testing. Thanks to Not_a_Number for the catch.