if ($#new != $#old) { #was a element removed (same num elements if not)

use Fcntl ':flock';

# Usage: unsubscribe ('email@address.com');
sub unsubscribe {
my $email = shift;

# Read email file into $data.
open (DB, "< $email_db_path$email_db") or die "Can't read open: $!";
flock (DB, LOCK_SH);
read (DB, my $data, -s DB);
close DB;

# Check to see if the string $email followed by a pipe is in this file.
# and we see that we can remove the line.
if ((index ($data, "$email|") >= 0) and ($data =~ s/^$email.+\n//m)) {
open (DB, "> $email_db_path$email_db") or die "Can't write open: $!";
flock (DB, LOCK_EX);
print DB $data;
close DB;
print "Location: $redirect_unsub\n\n";
}
# If index returned -1, that means it didn't match.
else {
error ("Could not unsubscribe!");
return;
}
}

# Usage: subscribe ('email@address.com', 'name');
sub subscribe {
my ($email, $name) = @_;

# Read email file into $data.
open (DB, "< $email_db_path$email_db") or die "Can't read open: $!";
flock (DB, LOCK_SH);
read (DB, my $data, -s DB);
close DB;

# Check to see thtat the user already isn't subscribed.
if ((index ($data, "$email|") >= 0) and ($data =~ s/^$email\|.+\n//m)) {
error ("Already subscribed!");
return;
}

# Nope, add the user in.
open (DB, "> $email_db_path$email_db") or die "Can't write open: $!";
flock (DB, LOCK_EX);
print DB $data, "$email|$name\n";
close DB;
print "Location: $redirect_sub\n\n";
}

sub main {
# --------------------------------------------------------
$|++; # Flush Output.
my %in = &parse_form;
my $action = $in{'action'};
my $email = $in{'email'};
my $name = $in{'name'};
($email =~ /.+@.+\..+/) or &error ("The email address: '$email' doesn't look like a real email address.") and return;
($name =~ /[A-Za-z]+/) or &error ("Please enter a name as well as an email address.") and return;

CASE: {
($action eq "subscribe") and do { &subscribe ('$email', '$name'); last CASE; };
($action eq "unsubscribe") and do { &unsubscribe('$email'); last CASE; };

&display_form();
};
}

my @return; foreach (@array) { push @return, print } @return;

@new_list = map { "prefix$_" } @old_list

@new_list = grep { substr($_, 0, 1) ne 'a' } @old_list

%hash = map { ($_ => 1) } @list;

if ($hash{$value}) { ... }