Not exactly sure what you want, but maybe something like (needs 5.10+ for \K):

c:\@Work\Perl>perl -wMstrict -le
"use 5.010;
 ;;
 my %xlate = (qq{\n} => 'n',  qq{\t} => 't',  qq{\r} => 'r');
 my $interp = join '', keys %xlate;
    $interp = qr{ [\Q$interp\E] }xms;
 print $interp;
 ;;
 my $pre = qr{ AD | MAIN }xms;
 ;;
 for my $s (qq{AD\thomas}, qq{MAIN\nancy}, qq{nancy}) {
   printf qq{'$s' -> };
   (my $t = $s) =~ s{ $pre \K ($interp) }{$xlate{$1}}xmsg;
   print qq{'$t'};
   }
"
\?^msx: [\      \
] )
'AD     homas' -> 'ADthomas'
'MAIN
ancy' -> 'MAINnancy'
'nancy' -> 'nancy'
[download]