You "need" to help yourself. Read Markup in the Monastery AND perldoc perlretut and perldoc perlre. This is NOT code-a-matic -- even when you post unintelligible nonsense.

Please format each code, data and sample output within a SEPARATE <c> </c> (code) block to make your question easy to understand.

Your attempt at substitution is

1. wrong
2. un-neccessarily complex
3. incomplete (post compileable code; show how you're populating $string).

UPDATE: Fixed borked codetag example. TY, MLX!

The flip-flop operator can help:

use strict;
use warnings;
while(<DATA>){
    next if m|^/\*.*\*/|;
    print if /^OT;$/../^RT;$/;
}

__DATA__
OT;
some line
/*asas*/        ##here comments should be removed
/*asdas*/  #OT;.....RT;should be replaced with OT;.....DT;OT.......RT;
some line
RT;

some lines 
somlelines

OT;
some line
/*asas*/
/*asdas*/
some line
RT;
[download]

2nd comment since OP has not responded to the previous nodes

#! /usr/bin/perl -w
use 5.018;

my $string = "OT;
 some line
 /*asas*/        # here C-style comments should be removed
 /*asdas*/       #OT;.....RT;should be replaced with OT;.....DT;OT....
+...RT; *note1
 some line
 RT;
 
 some lines 
 somlelines
 
 OT;
 another line
 /*asas*/
 /*asdas*/
 yet another line
 RT;";

=head

first, my note:
# /me does not understand WTF the OP's comment above means 

and now, OP's spec:

 what i need is like 
 
 OT;
 some line1
 some line2
 DT;
 OT;
 some line1
 some line2
 RT;
 
 some lines3 
 somlelines4
 
 OT;
 some line5
 some line6
 DT:
 OT;
 some line5
 some line6
 RT;

=cut

my $regex1 = qr([/][*]);
my $regex2 = qr([*][/]);

$string =~ s/$regex1.+?$regex2//gs;
say $string;

=head output

C:\> junk_check.pl
OT;
 some line
         # here C-style comments should be removed
        #OT;.....RT;should be replaced with OT;.....DT;OT.......RT; *n
+ote1
 some line
 RT;

 some lines
 somlelines

 OT;
 another line


 yet another line
 RT;

C:\>

=cut
[download]

The above does part of what you ask, without deleting the Perl-style comments, but /me does NOT understand your spec re inserting DT; (or DT: as in an instance in your example). Perhaps, if not in your own interest, in the interest of future readers, you will clarify the intent (and the probable typo) by an explicit explanation of when DT; should be inserted.