my ($Results1, $Results2);
&DoList($List, $Results1, $Results2, $tmp, $var);
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If you want the subroutine to modify its parameters, you have to modify elements of @_ directly, not taking a copy:

sub DoList {
    my $List = $_[0]; # first element in @_
    my ($tmp, $var) = @_[3,4];
    $_[1]    = `$List $tmp`;
    $_[2]    = `$List $var`;
}
[download]

See perlsub, in the DESCRIPTION section just after the first code examples: "... a function is free to do in-place modifications of @_ and change its caller's values".

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}