It would be unusual to find strings which did match that regex since it's a schema part of a URL and then a bunch of uppercase letter Ss with a space in between and a space at the end. But you haven't shown your input, so who knows what you are really trying to achieve? I could guess that you meant to put a backslash before the S and maybe not have those spaces in there but instead here's an SSCCE which you can tailor to your needs should you need to refine your question.

use strict;
use warnings;

use Test::More;

my @good = (
    'http:// S ',
    'We will all go to http:// SSSSSSSS ',
);

my @bad = (
    'foo',
    'http:// foo ',
    'https:// SSS ',
    'http:// SSS',
    'http://SSS ',
    'HTTP:// SSS ',
    'http://  SSS ',
    "http://\tSSS ",
    "http:// SSS\t",
);

my $re = qr{http:// (S+) };

plan tests => @good + @bad;

for my $str (@good) {
    like ($str, $re, "$str matched");
}
for my $str (@bad) {
    unlike ($str, $re, "$str not matched");
}
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