use Modern::Perl;

my $string = 'ATATTTATATTAT';

my ( $subStr1, $theRest1 ) = unpack '(a3)(a*)', $string;
say $subStr1;
say $theRest1, "\n";

my ( $subStr2, $theRest2 ) = $string =~ /(.{3})(.+)/;
say $subStr2;
say $theRest2;