Please remember to "use strict;" and "use warnings;". I reproduced the error "Modification of a read-only value" based on the incomplete code fragments you provided by commenting out "use strict;". Once you apply strict you get a more informative message that might have avoided this post.

use strict;
use warnings;

my $d = 3;

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

eval {foo($d);};
print $@; # prints informative message provided by strict
foo(\$d);
print "$d\n";

__END__
Can't use string ("3") as a SCALAR ref while "strict refs" in use ...
4

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Ron

In reply to Re: Modifying passed-in variables by mr_ron
in thread Modifying passed-in variables by mike65535

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