local $_  = $your_input_string;
while ( /ATG/g ) {
   my $start = pos() - 3;
   if ( /T(?:AA|AG|GA)/g ) {
     my $stop = pos;
     print $start, " ", $stop, " ", $stop - $start, " ", 
       substr ($_, $start, $stop - $start), $/;     
   }
}
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