I'd make a little script with the first two lines you showed but add two more:

my $value = '%22Happy%20Birthday%21%22';
$value =~ tr/+/ /;
$value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C",hex($1))/eg;
print "$value\n";
[download]

Try running that, it should show you the effect of your code, if not the mechanism.

Phil