my $http='http://domain.com/';
my $sample='<img src="pics/local.jpg" alt="" />
<img src="http://remote.com/pics/remote.jpg" />
more confusion, just in case: 3:00pm 12/12/12
other urls <a href="http://fake.com">
http://fake.com</a>';

#-- THIS -------------------
$sample=~s{(img src=")(\w?)[^:]}{$1$http$2}g;
# something from my book, POSIX ? not very clear
#-- OR ---------------------
$sample=~s/(img src=")(\w.+?)(:?)/"$1$http$2"if(!$3)/eg;
#-- END --------------------