my $spec="11-29,39-72,99-100,103,107";
my $input = "The quick red fox jumped over the lazy brown dog.";
# break spec into set of offsets and lengths

# for each offset and length...
for (my $idx=0 .. $#specs) {
   my ($input, $offset) = # how did I store them?

   #...extract a substring...
   my $substring = substr($input, $offset, $length);

   #...then print the result
   print "<$substring> is substr(...,$offset,$length)\n";
}