Modification of read-only value error

sk has asked for the wisdom of the Perl Monks concerning the following question:

Dear monks,

Here is a code snippet that basically rollsup a line until the keyword end; is reached then it prints it out. I believe the approach i am taking is called a closure (please correct me if i am wrong).

#!/usr/bin/perl

use strict;
use warnings;

my $dummy = "";
my $rollup = combine("");
my $oneline;

while (<DATA>) {
    $rollup->($_);

    # once end is reached store it in a variable    
    # reset variable-in-sub for next iteration
    if (/end;/) {
        # breaks if I remove $dummy and replace it with ""
        $oneline = $rollup->($dummy);
        $rollup = combine("");
        print $oneline,$/;
    }
}


sub combine {
    my $concat = shift;
    return sub {
        chomp($_[0]);
        $concat .= $_[0];
        return ($concat);
    }
}
[download]

__DATA__
line1;
line2;
line3;
end;
line4;
line5;
end;
[download]

Expected Output:

line1;line2;line3;end;
line4;line5;end;
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The above code works just fine but when I use $oneline = $rollup->(""); instead of $oneline = $rollup->($dummy);

I get the following error -

Modification of a read-only value attempted at closure line 26, <DATA>
+ line 4.
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Could you please explain to me what is the read-only value here?

I know i could do this whole step in a while loop and concatenate until end; is reached but wanted the flexibility in a sub and this also helps me learn more about closures :)

Thanks very much!

Update: I have changed the title and took out the word closure as the problem is nothing to do with that.

Comment on Modification of read-only value error Select or Download Code

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Re: Modification of read-only value error when using closure by Zaxo (Archbishop) on Aug 24, 2005 at 04:59 UTC
The error is from attempting to modify $_[0] when the argument is a string literal, `$rollup->(" ")` rather than a variable `$rollup->($dummy)`. The string literal is a constant, not an lvalue. After Compline, Zaxo	[reply]
Re^2: Modification of read-only value error when using closure by sk (Curate) on Aug 24, 2005 at 05:12 UTC
Thanks Zaxo!!! I was under the impression that args are sent as values through @_ and thought that this problem was something to do with closures. I went back and tried a chomp on simple sub and the same happened. I checked `perldoc perlsub` and i got this info `Any arguments passed in show up in the array @_. Therefore, if you ca +lled a function with two arguments, those would be stored in $_[0] an +d $_[1]. The array @_ is a local array, but its elements are aliases + for the actual scalar parameters. In particular, if an element $_[0 +] is updated, the corresponding argument is updated (or an error occu +rs if it is not updatable).` [download] Now it is clear to me! thanks again! cheers SK	[reply] [d/l] [select]