Well, in the first line, you have a numeric literal. In numeric literals, one can use underscores to separate non-empty sets of digits. (Two underscores in a row, or a trailing underscore, gives a warning - a leading underscore means it's unparsable). The underscores are ignored.

The second line has a string literal. It will be kept as is.

The third line adds a number and a string. In that case, Perl numifies the string by calling atoi or atol. Which will stop parsing at the first underscore (a warning will be given). That's not a bug - that's just the way how it is.