sub blah
{
    my %args=(@_);
    foreach my $needed (qw( foo bar baz )) {
        exists $args{$needed} or die "blah() needs a $needed parameter";
    }
    # ....
}

blah(foo => 42, bar => "don't forget this", baz => 'oh!');
blah(foo => 42, baz => 'oh!'); # <-- will die()