c:\@Work\Perl>perl -wMstrict -le
"my $s = 'XXXxxxTGAxxTGAxxxxxxx';
 while ($s =~ m{ \G (?{ print qq{trying a match at offset }, pos $s }) (?: \w\w\w)*? (TGA) }xmsg) {
     print qq{matched TGA beginning at offset $-[1]};
     }
"
trying a match at offset 0
matched TGA beginning at offset 6
trying a match at offset 9