#!/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";