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;
                    # ...
                }
            );
        }
    );