my @strings = (
  'http://foo.example.org/',
  'mms://bar.example.org/',
  'https://foobar.example.org/',
  'http:///tmp/foobar.txt',
);

for my $str ( @strings ) {
  if ( substr($str,0,8) eq 'http:///' ) {
    print "Treffer 'http' bei $str\n";
  }
}