my @available_things  = qw/ant bat cat dog /;
my @requested_things  = qw/bat rat/;
my @not_available_requested_things = 
  map {grep(!/$_/,@available_things)} @requested_things;
print "available: @available_things\n";
print "requested: @requested_things\n";
print "requested, not available: @not_available_requested_things\n";