use warnings;
use strict;

$_ = 'AAGCTT';
while (/(.)/g) {
    my $letter = uc $1;
    print $letter;
    my $len = length($letter);

    my $sub = "";
    if    ( $letter eq "A" ) { $sub = "T"; }
    elsif ( $letter eq "T" ) { $sub = "A"; }
    elsif ( $letter eq "G" ) { $sub = "C"; }
    elsif ( $letter eq "C" ) { $sub = "G"; }
    print "  $len\t$sub\n";
}    # close while

__END__

A  1	T
A  1	T
G  1	C
C  1	G
T  1	A
T  1	A