This is still not a piece of code that I can run on my machine and see your problem. Distil your code down to a stand alone sample that exhibits the problem. There are too many unknowns in the code you have provided. I say again, read I know what I mean. Why don't you?.

Note that your regex is probably much better done as a split:

my ($Institution, $CourseNumber, $Professor, $Enrollment) = split '|',
+ $try;

[download]

Oh, hold on a moment, my esp cells are kicking in. Here is the sample you should have posted:

use warnings;
use strict;

my @courselist;
my @text = 'UM|CS 34|Smith|34';
my $courseindex = 0;

foreach my $try (@text){
    my ($Institution, $CourseNumber, $Professor, $Enrollment) = split 
+'|', $try;
    
    $courseindex = $courseindex + 1;
    $courselist[$courseindex] =
        [$Institution, $CourseNumber, $Professor, $Enrollment];
};

@courselist =  sort { $a->[0] cmp $b->[0] || $a->[1] cmp $b->[1]} @cou
+rselist;

[download]

And the answer is that you preincrement $courseindex so the first element is placed at index 1, but Perl generally starts arrays at 0 and trying to dereference aan undefined value in the sort (element 0) is generating your error.

Your code would be better written as:

use warnings;
use strict;

my @courselist;
my @text = 'UM|CS 34|Smith|34';

foreach my $try (@text){
    my ($Institution, $CourseNumber, $Professor, $Enrollment) = split 
+'|', $try;
    
    push @courselist, [$Institution, $CourseNumber, $Professor, $Enrol
+lment];
};

@courselist =  sort { $a->[0] cmp $b->[0] || $a->[1] cmp $b->[1]} @cou
+rselist;

[download]
DWIM is Perl's answer to Gödel

In reply to Re^2: How to fix error: Modification of a read-only value by GrandFather
in thread How to fix error: Modification of a read-only value by Gnat53

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.