If you want to modify $x and $mod, your final calling convention will not work. The array reference creation syntax there actually copies the values from $x and $mod, it doesn't create references to them:

sub change {
    my $arr = shift;
    $_++ for @$arr;
}
my $x = 1;
my $y = 2;

change([ $x, $y ]);
print "($x) ($y)\n";
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If you want to modify the values themselves, you'll have to do something like:

sub change {
    my $arr = shift;
    $$_++ for @$arr;
}

my $x =  1;
my $y = 2;
change([ \$x, \$y ]);
print "($x) ($y)\n";
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which is exactly what you have. Or you could say change([ \( $x, $y ) ]);, which is my preference.