#!/usr/bin/perl -w

use strict;

print "Letter? ";
chomp(my $letter = <STDIN>);
open DATA, "books.txt" 
	or die "open(books.txt) returned: $!\n";
print grep {/^$letter/io} <DATA>;

##</code><code>##

perl -e 'print grep {/^a/io} <>' /usr/dict/words