'A val that is guaranteed not to be present in any list'

ow! Please use out-of-band signaling. Safer, Simpler, Prettier.

Method 1, return value + lvalue param:

my $fetch = sub {
    my ($fh) = @_;
    return if eof $fh;
    $_[1] = scalar <$fh>;
    return 1;
};

...

while ($next->(my $item)) {
    print "$item\n";
}
[download]

Method 2, undef vs reference:

my $fetch = sub {
    my ($fh) = @_;
    return if eof $fh;
    return \(scalar <$fh>);
};

...

while (my $ref = $next->()) {  
    print "$$ref\n";
}
[download]

Method 3, empty list vs non-empty list (like each):

my $fetch = sub {
    my ($fh) = @_;
    return if eof $fh;
    return scalar <$fh>;
};

...

# RIGHT: while (my ($item) = $next->())
# WRONG: while (my $item = $next->())

while (my ($item) = $next->()) {  
    print "$item\n";
}
[download]

Update: Added second and third method.