use strict;
use warnings;

my $in = shift || 'GATATTTTTTT';

$_       = $in;
my $last = substr $_, -1;

if ( length > 2  and  substr($_, -2, 1) eq $last ) {
   chop while substr($_, -1) eq $last;
   $_ .= $last . $last;
}

print "in : $in\n";
print "out: $_\n";