$_ = 'TATATATATATA';
while (/(?=TATA)/g) {
    print "Matched 'TATA' at position ", pos, "\n";
};;
Matched 'TATA' at position  0
Matched 'TATA' at position  2
Matched 'TATA' at position  4
Matched 'TATA' at position  6
Matched 'TATA' at position  8

$_ = 'TATATATATATA';
while (/(?<=TATA)/g) {
    print "Matched 'TATA' at position ", pos, "\n";
}
Matched 'TATA' at position  4
Matched 'TATA' at position  6
Matched 'TATA' at position  8
Matched 'TATA' at position  10
Matched 'TATA' at position  12