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.
[download]

still confusing...