From the Camel Book (4^th Edition, 2012), page 328:

As the last character of a prototype, or just before a semicolon, you can use _ in place of $. If this argument is not provided, the current $_ variable will be used instead. For example:

sub mymkdir(_;$) {
  my $dirname =   shift();
  my $mask = @_ ? shift() : 0777;
  my $mode = $mask &~ umask();
  ...
}

mymkdir($path, 01750);
mymkdir($path);
mymkdir();    # passes in $_
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Hope that helps,