Re: Closure producing 'Modification of read-only value' error

The $_ of your for loop is aliased to the read-only strings qw(one two three). The while loop inside the sub overwrites $_, hence the "Modification of a read-only value attempted" error. If you add the line:

    local $_;
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inside your sub get_number it works. Generally, mangling $_ inside a subroutine is poor form.

Update: a cleaner way to ensure the sub plays well with others is to simply change the while loop to use a lexical rather than the implicit $_:

      while (defined(my $d = <DATA>)){
        chomp $d;
        my ($str, $num) = split /\|/, $d;
        $hash_ref->{$str} = $num;
      }
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Re^2: Closure producing 'Modification of read-only value' error by nobull (Friar) on Jun 11, 2005 at 11:04 UTC
No, don't do that. Never `local $_` in a subroutine. If you want to use the magic assignment you should do `local *_`. Of course you have to unpack anything you want from `@_` first. If you get into the habit of doing `local $_` inside you subroutines it will seem to work fine - maybe for years. Then one day someone will call one of your subroutines inside a `for()` loop that is iterating over the values of a tied array or hash. Very odd things will start happening in their program.	[reply] [d/l] [select]
Re^3: Closure producing 'Modification of read-only value' error by tlm (Prior) on Jun 11, 2005 at 16:39 UTC
What about the implicit localizations of `$_` done by `map`, `grep`, `for`, etc.? Do they do The Right Thing®? I think it is surprising (hence bad) that the `while ( <FH> )` idiom doesn't do an implicit localization of `$_` like the looping constructs listed above. Is there a fundamental reason for this, or is it due to a severe tuit shortage? the lowliest monk	[reply]
Re^4: Closure producing 'Modification of read-only value' error by nobull (Friar) on Jun 11, 2005 at 19:09 UTC
`map`, `grep`, `for` etc do the right thing. The magic while construct does not make `$_` and alias - it assigns to it. This is fundamentally different. That said, it bugs me that it doesn't do implicit localization.	[reply] [d/l] [select]
Re^3: Closure producing 'Modification of read-only value' error by kaif (Friar) on Jun 13, 2005 at 02:03 UTC
I'm sorry, maybe I'm dense, but what exactly is different when you iterate over a tied array or hash. What are these "very odd things" and why won't a simple `local $_` do? This is very interesting to me.	[reply] [d/l]