my   @part = ( 'http://example.net/app' );
push @part, 'admin'  if $is_admin_link;
push @part, $subsite if defined $subsite;
push @part, $mode;
push @part, $id      if defined $id;
push @part, $submode if defined $submode;

my $uri = join '/', @parts;

##</code><code>##

# [updated: originally had `'admin' ? $is_admin_link : ()`]

my @part = (
	'http://example.net/app',
	( $is_admin_link   ? 'admin'  : () ),
	( defined $subsite ? $subsite : () ),
	$mode,
	( defined $id      ? $id      : () ),
	( defined $submode ? $submode : () ),
);

my $uri = join '/', @part;

##</code><code>##

$a = undef;
print $a ? 'true' : 'false';  # prints 'false'
print $a;                     # prints '' and warns
$a = 0;
print $a ? 'true' : 'false';  # prints 'false'
print $a;                     # prints '0'
$a = 'abc';
print $a ? 'true' : 'false';  # prints 'true'
print $a;                     # prints 'abc'

# whereas

$a = undef;
print !!$a ? 'true' : 'false';  # prints 'false'
print !!$a;                     # prints '' without warning
$a = 0;
print !!$a ? 'true' : 'false';  # prints 'false'
print !!$a;                     # prints ''
$a = 'abc';
print !!$a ? 'true' : 'false';  # prints 'true'
print !!$a;                     # prints 1

##</code><code>##

@_ = ( 'a' ) x 4;
print join ':', @_;  # prints 'a:a:a:a'
print scalar @_;     # prints '4'

##</code><code>##

@_ = ( 'a' ) x 1;
print join ':', @_;  # prints 'a'
print scalar @_;     # prints '1'

@_ = ( 'a' ) x 0;
print join ':', @_;  # prints ''
print scalar @_;     # prints '0'

##</code><code>##

my @part = (
	'http://example.net/app',
	( 'admin'  ) x!! $is_admin_link,
	( $subsite ) x!! defined $subsite,
	$mode,
	( $id      ) x!! defined $id,
	( $submode ) x!! defined $submode,
);

my $uri = join '/', @parts;