Yes, that's why you need to figure out the actual line number. "123" was just an example. You can determine the line number with l Engine::_Execute ... lines that are breakable will show with a colon (:) after the line number in the debugger. Hence, if you see this:
100 sub Engine::Execute
103: foreach (@commands)
105: $cmd = $_;
106: if($cmd =~ /INITIAL/i)
... then you would type b 106 $cmd =~ /INITIAL/i
Again, these numbers are just examples. You need to determine the actual line number for yourself with l Engine::_Execute!