$ perl -e'
my $test = "xTx\nxxTxxT";
my $rx = qr/(?=(x...T))/s;
my @matches = $test =~ /$rx/g;
print "match #", $_ + 1, ":\n$matches[$_]\n" for 0 .. $#matches;
'
match #1:
x
xxT
match #2:
xTxxT