Tokenization incorrect when using underscore in function name and @_

[alexr00]

From @gilsanx in microsoft/vscode#83321

• VSCode Version: 1.39.2
• OS Version: Fedora 20

Steps to Reproduce:

1. Have a perl function that has an underscore in its name. Example: s_expand()
2. Pass @_ as parameter. s_expand(@_)
3. The syntax is broken until another character breaks the highlight, for instance, another underscore.

sub myExample()
{
  s_expand(@_);
}

sub fixHighlight()
{
  #perl comment_
}