On the top of page 23 in my Camel 2, it reads

There is no "elsunless" though. This is generally construed as a feature.

I believe the authors meant that there is no keyword that means "elsif (not EXPR)" in the way that "unless (EXPR)" means "if (not EXPR)." You can, however, use elsif and else with unless:

foreach $a ( 0,1,2 ) {
  unless ( $a ) {
    print "a is 0 or undef\n";
  }
  elsif ( $a == 1 ) {
    print "a is 1\n";
  }
  else {
    print "a equals $a\n";
  }
}

__END__
a is 0 or undef
a is 1
a equals 2
[download]

I hope this clears things up for you.

--sacked