Some further points on top of what Zaxo said:

A minor one: habitually limit splits unless you explicitly want "any number" of results. In this case, you're not interested in anything past the 3rd element, so limit it to 4 columns.

And you're doing too much work. If you do not explicitly want the smallest free UID assigned to the new account, you can roll the entire thing into the first loop that reads the passwd:

my $g_user_id = 200;
while (<PWD>) {
   my ($accountname, $accountid) = (split /:/, $_, 4)[0,2];
   die "This account name already exists!\n" if $accountname eq $usern
+ame;
   $g_user_id = $accountid + 1 if $accountid > $g_user_id;
}
die "Ack, too many UIDs assigned.\n" if $g_user_id > 65535;
[download]

Note I removed the chomp since the last record on the line is of no interest anyway.

But, yes, your premise is absolutely correct. I believe you will enjoy the following read: Teach Yourself Programming in Ten Years. It pretty much sums up the entire topic.

Makeshifts last the longest.