This crops up often enough. Like mentioned, you shouldn't operate on $_ because it is an alias to the elements of the original data structure, and as such modifies them. Here is what cou can do:

my $original = [qw(alpha beta gamma)];
my @duplicate;

for (0..$#$original) {
  my $temp = $original->[$_];
  $temp = rand();
  push @duplicate, $temp;
}

print "Original: $_\n" for (@$original);
print "Mofified: $_\n" for (@duplicate);
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Which gives the expected, and desired:

Original: alpha
Original: beta
Original: gamma
Mofified: 0.421875
Mofified: 0.108734130859375
Mofified: 0.69537353515625
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Update: Just to make things a bit more clear. I can obviously see that the original question is conserned with subroutine arguments being references. I merely expanded on the code that was attached as to not complicate the situation. Bull-blown examples with subroutines had already been posted prior to my reply. So fast to --?

Update: you might be also interested in this node, global $_ behavior which discusses up to some extent aliasing.