package rdw;
use Carp;

sub TIEHANDLE {
    my ($class, $file) = @_;
    open ( my $file , '<', $file ) or croak "Unable to open $file : $!";
    return bless \$file , $class;
}

sub READLINE {
    my $self = shift;
    my ($length, $record);
    read $$self, $length, 1;
    return undef if eof $$self;
    read $$self, $record, $length;
    return $record;
}
42;

# and a script that uses it

#!/usr/bin/perl
use strict;
use warnings;
use rdw;
tie *fh, 'rdw', 'foo.rdw' or die "Unable to tie : $!";
while ( <fh> ) {
    print "$_\n";
}
__END__
# foo.rdw - outputs "a\nab\nabc\nabcd\n" as desired
1a2ab3abc4abcd