Without knowing the contents of $file or how you're checking that the "text remains untouched" I don't think that anyone can tell you what you're doing wrong with any certainty. But just to show you that your code does indeed "work":

#!/usr/bin/perl

use strict; use warnings;

my $str = "This is a string with a URL like http://www.example.com in 
+it";

print "$str\n";
$str =~ s/http\S+/ /ig;
print "$str\n";
__END__
This is a string with a URL like http://www.example.com in it
This is a string with a URL like   in it
[download]

Perhaps it would help you to try to reduce the problem to something simple like the above?