I get similar behavior on Win2000. The fork appears to mess with the file handle:

#!/usr/bin/perl

use strict;
use warnings;

my $x=0;
while (my $computer=<DATA>) {
       print "computer: [$computer]";
       chomp($computer);
       $x++;   
       
    my $pid = fork();
    if ($pid) { 
        print "Parent $$ got PID $pid from fork($$ $computer)\n"; 
        sleep(1);
    }else { 
          print "\t$$ $computer\n";
          exit(0);         
    }

       if($x > 4) {
          do { $pid=wait() } while ($pid != -1);
          $x=0;
       }
}

my $pid;
do { $pid=wait() } while ($pid != -1);
print "wait() ed until $pid was returned.\n";
   
close(DATA);

__DATA__
Computer1
Computer2
Computer3

# Output
computer: [Computer1
]Parent 1782987 got PID -1853375 from fork(1782987 Computer1)
        -1853375 Computer1
computer: [
]Parent 1782987 got PID -1852747 from fork(1782987 )
        -1852747
computer: [Computer2
]Parent 1782987 got PID -1805143 from fork(1782987 Computer2)
        -1805143 Computer2
computer: [
]Parent 1782987 got PID -1804643 from fork(1782987 )
        -1804643
computer: [Computer3
]Parent 1782987 got PID -1898099 from fork(1782987 Computer3)
        -1898099 Computer3
wait() ed until -1 was returned.
[download]

The problem goes away if you slurp the input first, like:

for my $computer (<DATA>) {
...
[download]