Ubuntu Unleashed 2013 Edition: Covering 12.10 and 13.04, Eighth Edition

unless

unless works just like if, only backward. unless performs a statement or block if a condition is false:

unless ($name eq "Rich")        {  print "Go away, you're not allowed in here!\n";}

Note

You can restate the preceding example in more natural language, like this:

print "Go away!\n" unless $name eq "Rich";