Grep is not obscure. It is much clearer than dumb loops that are really just low level implementations of list operations.

#!/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>;
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Of course, I kind of like something like

perl -e 'print grep {/^a/io} <>' /usr/dict/words
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Try it, you'll like it too!