# ...
107    sub foo {
108        my $x;
109        $x = 43;
110 =>     die "invalid x!" unless $x == 42;
111        print "Yay!"
112        # ...

##</code><code>##

109        $x = 43;
           # When I hit "save", foo is recompiled. If line numbers
           # have changed, breakpoints are updated.