#! /usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

my $dna = join q(), qw( TGA TGA ATG AGA );
for my $regex (qr/(\w\w\w)*?TGA/,
               qr/^(\w\w\w)*?TGA/,
               qr/\G(\w\w\w)*?TGA/,
              ) {
    while ($dna =~ /$regex/g) {
        say "TGA with $regex: ", pos $dna;
    }
}