Argument "A" isn't numeric in numeric eq (==)
####
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
##
##
my %conv = (
A => 'T',
T => 'A',
G => 'C',
C => 'G',
);
$_ = 'AAGCTT';
while (/(.)/g) {
my $letter = uc $1;
print $letter;
my $len = length($letter);
print " $len\t$conv{$letter}\n";
}