hehe, on the idea of more "perl looking", in my opinion:

#/usr/bin/perl -w
use strict;
print "Letter: ";
chomp(my $letter = <STDIN>);
open(DATA, "books.txt") or die "error opening file $!";
while(<DATA>) {
        if (/^$letter/i) {
                print;
        }
}
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charlie schmidt
ishamael@themes.org
www.diablonet.net/~ishamael/