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

...

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

##</code><code>##

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

...

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

##</code><code>##

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";
}