#! /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; *note1
 some line
 RT;

 some lines
 somlelines

 OT;
 another line


 yet another line
 RT;

C:\>

=cut