chomp the input from /etc/passwd to remove the newlines, splitting into an array is much cleaner in my opinion. Checking your open calls and using strict and warnings is also good practice.

#!/usr/bin/perl -w

use strict;

open INPUT, "</etc/passwd" or die "Can't open file #!\n";

my @lines;

while (<INPUT>) {

    chomp;
    @lines = split /:/;

    write();
}

format STDOUT_TOP =
NAME                    ID
----                    --
.

format STDOUT =
@<<<<<<<<<<<<<<         @<<<<<<<<<<<<<<<<<<
$lines[0], $lines[4]
.
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Also, what systems is it failing on, what errors does it give?.