# Inefficient!

my $s = 'ABCDEFGHIJKLMNOPCDEFQRST';

my $first  = 'C';
my $middle = 'DE';
my $last   = 'F';

my $spos = -1;
for (;;) {
   $spos = index($s, $first, $spos+1);
   last if $spos < 0;

   my $epos = $spos + length($first) - 1;
   for (;;) {
      $epos = index($s, $last, $epos+1);
      last if $epos < 0;

      my $len = $epos - $spos + 1 - length($first) - length($last);
      if (substr($s, $spos+1, $len) eq $middle) {
         print("Match at $spos\n");
      }
   }
}