Just for fun, using an array slice instead of a loop (quickly tested under the Perl debugger):

main::(-e:1):   42
  DB<1> $input = "The quick red fox jumped over the lazy brown dog.";

  DB<2> $spec = "3-6,9-13,17-20";

  DB<3> $spec =~ s/\-/../g;

  DB<4> @in = split //, $input;

  DB<5>  print "@in";
T h e   q u i c k   r e d   f o x   j u m p e d   o v e r   t h e   l 
+a z y   b r o w n   d o g .
  DB<6> print  @in[eval ($spec)];
 qui red  jum
  DB<7>
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Just 3 lines of code besides the initializations.

Now, of course, since array subscripts start at 0, I am counting the letters from 0. If we need counting letters from 1, not 0, we can just add an empty element at the beginning of the array:

  DB<7> unshift @in, "";

  DB<8> print  @in[eval ($spec)];
e quk redx ju
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