If you're intent on modifying the argument, why not do it directly instead of using local globs?

@array = (10,20);
Double(@array);
print "@array";

sub Double {
foreach my $element (@_) {
$element *=2;
}
}
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@_ is an alias for the arguments sent, and the foreach loop makes $element an alias for each element of @_ in turn, so this works out and is reasonably clearer.