But that means that the regex could also be fixed without recourse to \G, by simply anchoring it to the start of the string: 
while ($dna =~ /^(\w\w\w)*?TGA/g)

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With ^ instead of \G, the regex would match only once, even with the /g modifier: because after a succesful match, the next match starts when the previous one ended, and ^ can't match there. With \G, though, you can get all the matches from the loop. 
#! /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;
    }
}

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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

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In reply to Re^2: Understanding a portion of perlretut by choroba
in thread Understanding a portion on the Perlretut by BlueStarry

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