#! perl
use strict;
use warnings;

while (my $dna = <DATA>)
{
    chomp $dna;
    print "\n\$dna = '$dna'\n";

    while ($dna =~ /(\w\w\w)*?TGA/g)
    {
        print 'Got a TGA stop codon at position ', pos $dna, 
              ', immediately following [', $1, "]\n";
    }
}

__DATA__
ATCGTTGAA
ATCGTTGAATGCAAATGACATGAC

##</code><code>##

 0:10 >perl 1476_SoPW.pl

$dna = 'ATCGTTGAA'
Got a TGA stop codon at position 8, immediately following [CGT]

$dna = 'ATCGTTGAATGCAAATGACATGAC'
Got a TGA stop codon at position 18, immediately following [AAA]
Use of uninitialized value $1 in print at 1476_SoPW.pl line 43, <DATA> line 2.
Got a TGA stop codon at position 23, immediately following []

 0:10 >

##</code><code>##

while ($dna =~ /\G(\w\w\w)*?TGA/g)

##</code><code>##

while ($dna =~ /^(\w\w\w)*?TGA/g)