use strict;
use warnings;
use 5.010;

my $str = 'aaaBBBcccTGA';
while ($str =~ /(?:\w\w\w)*?(TGA)/g) {
    say $1;
    say pos $str;
}

##</code><code>##

use strict;
use warnings;
use 5.010;

my $str = 'aaaBBBcccTGAddTGA';
while ($str =~ /\A(?:\w\w\w)*?(TGA)/g) {
    say $1;
    say pos $str;
}

--output:--
TGA
12

##</code><code>##

use strict;
use warnings;
use 5.010;

my $str = 'aaaBBBcccTGAdddTGA';
while ($str =~ /\A(?:\w\w\w)*?(TGA)/g) {
    say $1;
    say pos $str;
}