Sorry to bother you again, but your recent advice doesn't seem to help me. I've changed the code into this:

while (<>) {
        chomp;
        (@row) = split(/ /,$_);
        foreach $item (@row) {
                ($node,$value) = split(/->/,$item,2);
                $graph{$node} = $value;
        }
        sub depth {
                return max map { $_ and 1 + depth($graph{$_}) } @_;
                shift @_
}
        print depth(keys %graph);
        print "\n";
        @row = ();
        $graph = ""
}
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, but I still get the values wrong, even if I just try it on a few lines of my infile. Here is an example of my infile (9 lines), picked at random but in a sequence: ------------------------------------

1->14 2->3 3->1 4->5 5->3 6->3 7->9 8->9 9->10 10->6 11->10 12->10 13-
+>10 14->0 15->14 16->15 17->18 18->15 19->18 20->19 21->14
1->2 2->0 3->5 4->5 5->6 6->2 7->2 8->7 9->2
1->2 2->0 3->4 4->2 5->7 6->7 7->4 8->7 9->8 10->2
1->2 2->0 3->2 4->3 5->7 6->5 7->11 8->7 9->7 10->9 11->4 12->11 13->1
+2 14->15 15->11 16->2
1->2 2->0 3->2
1->2 2->0 3->2 4->5 5->3 6->5 7->6 8->6 9->8 10->9 11->12 12->10 13->2
1->2 2->0 3->5 4->5 5->2 6->2 7->6 8->2
1->3 2->3 3->4 4->0 5->4 6->5 7->6 8->6 9->8 10->11 11->8 12->11 13->1
+2 14->4
1->2 2->8 3->2 4->2 5->6 6->4 7->6 8->0 9->8 10->8
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Here is the output that I get:

9
7
6
9
9
13
10
10
7
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Then, if I try the script on a separate input file, containing just those 9 lines, I get the following output:

7
5
5
8
8
13
10
9
6
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It's confusing... Do you have any ideas?

It's confusing... Do you have any ideas?

Yes. Don't remove the use strict; use warnings; from the start of the script. It's there for a reason: it tells you what you are doing wrong. (Namely you're not clearing %graph, but rather $graph, which is a totally different variable, which you've never declared. use strict; catches that errors).

Perl 6 - second systems done right

This is how my code looks like when I'm using strict and warnings:

#!/bin/perl;
use strict;
use warnings;
use List::Util qw/max/;

my %graph;

while (<>) {
        chomp;
        (@row) = split(/ /,$_);
        foreach $item (@row) {
                ($node,$value) = split(/->/,$item,2);
                $graph{$node} = $value;
        }
        sub depth {
                return max map { $_ and 1 + depth($graph{$_}) } @_;
                shift @_
}
        print depth(keys %graph);
        print "\n";
        @row = ();
        %graph = ""
}
[download]

Here are the error messages that I get:

Global symbol "@row" requires explicit package name at count_PT.prl li
+ne 11.
Global symbol "$item" requires explicit package name at count_PT.prl l
+ine 12.
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still confusing...