Proposed change:
sub run(
*@args ($, *@),
int :exitcode = 0, # <---- addition
:$in = '-',
:$out = '-',
:$err = '-',
Bool :$bin = False,
Bool :$chomp = True,
Bool :$merge = False,
Str:D :$enc = 'UTF-8',
Str:D :$nl = "\n",
:$cwd = $*CWD,
Hash() :$env = %*ENV,
:$arg0,
:$win-verbatim-args = False
--> Proc:D)
