#!usr/bin/perl-w
my $seq="ATCATCATCATC";
my (%first,%second,%third);
my @trilet = $seq =~ /.../g; # Line 4
# Line 5 Perlsyn LOOP
foreach my $letters ('AT','TC','AC') { #init
    $first{ $letters }=0;
    $second{ $letters }=0;
    $third{ $letters }=0;# Line 9
}
foreach my $tri (@trilet) {
    # Line 13 perlfunc : substr
    $first{ substr $tri,0,2 }++;
    $second{ substr $tri,1,2 }++; # Line 14
    $third{  substr $tri,2,2 }++; # Line 15
}
foreach my $letters ('AT','TC','AC') {
    print" Letters At 1st & 2nd Positions:\n";
    print "  $letters=$first{$letters}; "; # Line 19
}
print "\n\n";
foreach my $letters ('AT','TC','AC') {
    print" Letters At 2nd & 3rd Positions:\n";
    print "  $letters=$second{$letters}; ";# Line 24   
}
print "\n\n"; # Line 25
foreach my $letters ('AT','TC','AC') {
    print" Letters At 1st & 3rd Positions:\n";
    print "  $letters=$third{$letters}; ";# line 28 
}
print "\n\n";# Line 30 
exit;