#!/usr/bin/perl -w
use strict;

sub NEXT { $_[0]->() }

sub imap (&$) {
    my ($transform, $it) = @_;
    return sub {
        my $next = NEXT($it);
        return unless defined $next;
        return $transform->($next);
    }
}

sub string_to_char_iter {
    my $string = shift;
    $string = reverse $string;
    sub { '' ne $string ? chop $string : undef }
}

my $string = join '', 'a' .. 'z';
my $iter = string_to_char_iter $string; 

my $uc_chars = imap { uc $_[0] } $iter;

while ( my $char = NEXT $uc_chars ) {
    print "$char\n";
}