use warnings;
use strict;

local $/;

my $data = <DATA>;
while ($data =~ /TATAAT[ACGT]+ATG([ACGT]+)(:?T(:?GA|AA|AG))/cgs) {
warn $1;
}

__DATA__
TATAATATTACAATGGATCATACAGTTAG
TATAATATTACAATGGATCATACAGTTAG
TATAATATT
ACAATGGATCATACAGTTAG

##</code><code>##

$ perl tst.pl
GATCATACAGT at tst.pl line 8, <DATA> chunk 1.
GATCATACAGT at tst.pl line 8, <DATA> chunk 1.
$