the value of $letter has a line ending on the end. try this:

print "Letter? ";

my $input = <STDIN>;

# get rid of line ending
chomp($input);

# take the first character and escape it if it is a possible metachara
+cter
my $letter = quotemeta substr $input, 0, 1;

open DATA, 'books.txt' or die "Couldn't open books.txt: $!";

my @data = <DATA>;

close DATA;

foreach my $data (@data)
{
    print $data if $data =~ /^$letter/i;
}
[download]