How to lie to the debugger about what its lines of code are

gistfile1.pl

# lie to the debugger about what the lines of code are                                                                                                                               
no strict 'refs';
my $line_num = 0;
foreach ( @{"::_<$filename"} ) { 

    # uncomment these to blow your f'in mind
    #if ( not defined ) {
    #    use Devel::Peek;
    #    warn "line number is $line_num";
    #    Dump($_);
    #}  
    # The debugger special cases the first value in ::_<$filename.
    # It's "undef" but sometimes contains some data about the
    # program. I don't know entirely what it is, but this solves
    # the "off by one" bug.
    next unless defined;    # thanks Liz! (why does this work?)
    my $line = $lines->[ $line_num++ ];
    next unless defined $line;    # happens when $_ = "\n"
    my $numeric_value = 0 + $_; 

    # Internally, the debugger uses dualvars for each line of
    # code. If it's numeric value is 0, then the line is not
    # breakable. If we don't include this, no lines in the
    # debugger are breakable.
    $_ = dualvar $numeric_value, $line;
}