I thought of the function index first, which gets the position of a string (or character, in this case).

my $seq = "XX**XXX!!!X**AB*!C*DXX*!!!XXX**XXX";
# The code presumes that it's a correct sequence.
# If you're getting the sequence from somewhere else you
# should check it to make sure it contains the right letters.
print "#" . index($seq, "A") . "-#" . index($seq, "D") . "\n";
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This is the easiest way; your method works too, but it would involve splitting up the string and operating on that, so using index is easier.

Are you given ABCD or are you given the position and length (or start and end) of the substring? Can the * and !'s occur anywhere? You should give a before and after sample. Something like this:

__DATA__
XXXXXXABCDXXXXXXXX
XX**XXX!!!X**AB*!C*DXX*!!!XXX**XXX
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Want to print:

10  characters added
XXXXXXABCD**!!!***!*XX*!!!XXX**XXX
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The following code may be a good starting point:

use strict;
use warnings;

my $match = 'ABCD';

while (<DATA>)
  {
  my $org = $_;
  defined (my $mutated = <DATA>) or die "Missing edited line";

  my $segment = substr $mutated, 0, index ($mutated, substr $match, 3,
+ 1) + 1;
  my $suffix = substr $mutated, index ($mutated, substr $match, 3, 1) 
++ 1;
  (my $pInsert = $segment) =~ tr/*!//cd;
  (my $pSegment = $segment) =~ tr/*!//d;
  print length ($pInsert) . " characters added\n";
  print "$pSegment$pInsert$suffix\n";
  }

__DATA__
XXXXXXABCDXXXXXXXX
XX**XXX!!!X**AB*!C*DXX*!!!XXX**XXX
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Note that some error checking should be added and that there are a few assumptions about the match string and what the X sequences can actually contain.

$_ = 'XX**XXX!!!X**AB*!C*DXX*!!!XXX**XXX';
/A[!\*]*B[!\*]*C[!\*]*D/g and print '#', pos() + 1 - length($&), '-#',
+ pos();
__OUTPUT__
#14-#20
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