Perl Regular Expression question

Hi,

This is probably basic enough, but I can't figure it out. I want to do a string replacement in Perl.

I have a file with the following line:

IPAdd = 1xx.xx.xx.xx

and I want to replace it with

IPAdd = 1yy.yy.yy.yy

However, IPAdd 1xx.xx.xx.xx could be any IP Address, so I am searching for IPAdd and I want to replace the entire line with IPAdd = 1yy.yy.yy.yy

What I have so far is:

my $text;
my $string;
my $replace;

$string = "IPAdd";
$replace = "IPAdd = 1yy.yy.yy.yy";
$text =~ s/$string/$replace/g;

However, this writes the line IPAdd = 1yy.yy.yy.yy 1xx.xx.xx.xx
Can anyone please point me in the right direction for this?
To sum it up, I want to search for part of a string and replace that string and all of that strings line with a new string.

Thanks.