Monks- I'm trying to modify directly, a subroutine's passed-in variable. I've distilled it to the following (but had to hand-type; I hope I didn't introduce new errors!) 
my $data = 3;

foo  {
  my ($data) = @_;
  $$data++;
}

[download]
Perl throws an error at the $$data++ line.

I've looked at some sample Perl scripts that appear to operate this way.

I've looked over Common Causes for "Modification of a read-only value attempted" but it hasn't helped me.

Thanks!

In reply to Modifying passed-in variables by mike65535

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