You're programming like a crazy man, with code going everywhere! Maybe if you pull out some of the extraneous material and things will start to make sense:

while(<PWD>)
{
    my($name,$passwd,$uid,$gid,$gcos,$dir,$shell)=split /:/;

    $uid=$UIDS{$name} if (defined $UIDS{$name});

    $line=join ':',$name,$passwd,$uid,$gid,$gcos,$dir,$shell;
    print TMP $line;
}
[download]

Also, why use 'uname' when 'hostname' is a much better tool for finding your hostname. Not to mention the Sys::Hostname module which does an even better job.