Well, we can make guesses at what you mean. But a little sample code like the following would allow us to answer your actual question rather than any one of the many we could invent for you.

use strict;
use warnings;

my $temp = 'Plugh';
my $str = <<STR;
To %ad or not to %ad,
that is the question.
Oh %ad on you.
STR

$str =~ s/%ad/$temp/g;
print $str;
[download]

Prints:

To Plugh or not to Plugh,
that is the question.
Oh Plugh on you.
[download]

If that isn't the answer to the question you were intending, please alter the sample code to illustrate what you really want to know. You ought to read I know what I mean. Why don't you? BTW. It was written for you.

Update: fix cut and paste problem