# Code using iterator:
    while(  <INPUT>  ) {
        process_line( $_ );
    }

# Code using call-back:
    File::ProcessLines(
        \*INPUT,
        sub { process_line($_) }
    );

##</code><code>##

    # A merge sort:
    my $r1= <$i1>;
    my $r2= <$i2>;
    while(  ! eof($i1)  &&  ! eof($i2)  ) {
        my $cmp= $r1 cmp $r2;
        print $cmp le 0 ? $r1 : $r2;
        $r1= <$i1>   if  $cmp le 0;
        $r2= <$i2>   if  0 le $cmp;
    }
    # ...

##</code><code>##

    File::ProcessLines(
        $i1,
        sub {   # sub1
            my $r1= shift(@_);
            File::ProcessLines(
                $i2,
                sub {   # sub2
                    my $r2= shift(@_);
                    if(  $r1 lt $r2  ) {
                        return_from_sub1_but_not_from_sub2;
                    # ...
                }
            );
        }
    );