% cat t0
use strict;
use warnings;

my $max_width = shift @ARGV;
my $prefix = "foo";
my $postfix = "0123456789";
my $stored_width;
tie my $limit, 'LazyScalar', sub { $max_width - $stored_width };

printf "%s%n%.*s\n", $prefix, $stored_width, $limit, $postfix;

# A simple tied class for deferred evaluation of a scalar's value
package LazyScalar {
    sub TIESCALAR {
        my($class, $cb) = @_;
        return bless \$cb, $class;
    }
    sub FETCH {
        my($self) = @_;
        return $$self->();
    }
};

% perl t0 6
foo012
% perl t0 7
foo0123
% perl t0 8
foo01234
%