G'day gelbukh,

Welcome to the Monastery.

"Sorry for a stupid question! But I want to learn: ..."

• Not a "stupid question"!
• ++ for "I want to learn".
• Here's a description of what's going on.

A '#' introduces a comment. See "perlsyn: Comments". So, when compiling, Perl sees your lines 9 to 12 as:

my $N= 


say "N = $N";
[download]

As Perl doesn't care about the intervening whitespace, that's basically:

my $N= say "N = $N";
[download]

Assignment is a right-to-left operation. See "perlop - Perl operators and precedence". That's a fairly long page; the parts most pertinent to your question are:

• The table near the end of the "Operator Precedence and Associativity" section.
• The "Assignment Operators" section.

So, Perl attempts to compile 'say "N = $N"' first. It hasn't evaluated the left side of the assignment ('my $N') yet, so it sees $N as a "Global symbol" and hence the error message.

There is a subtle point here that isn't immediately, or intuitively, obvious. A statement such as 'my $x = ...;' has two distinct parts:

• 'my $x' declares $x as a lexical variable at compile time.
• The whole statement 'my $x = ...;' is compiled at compile time; however, the actual assignment is a runtime operation.

I see a couple of posts showing how to get length and last index of an array. Another way to get the length is:

my $N= 0+@Org;
[download]

As a further tangential point, a "use VERSION" statement, where VERSION is 5.12 or greater, you get 'use strict;' automatically. I like to put the use VERSION statement first: if the user does not have a sufficiently high enough version, you might as well stop here rather than doing other (effectively) pointless processing. Accordingly, instead of

use strict;
use warnings;

use 5.020;
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I'd write

use 5.020;
use warnings;
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Just something to consider for future reference.

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