my $string = "<S>yada yada yada</S>";
print "Yes! $1\n" if $string =~ m{<S>(.*?)</S>};

### Here is an explanation of the regular expression

use YAPE::Regex::Explain;

print YAPE::Regex::Explain->new('<S>(.*?)</S>')->explain;
__END__
The regular expression:

(?-imsx:<S>(.*?)</S>)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  <S>                      '<S>'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    .*?                      any character except \n (0 or more times
                             (matching the least amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  </S>                     '</S>'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------