defined %hash          scalar keys %hash

defined @array         scalar @array

exists $array[$elt]    defined $array[$elt]
       (and/or)        0 <= $elt && $elt < @array

delete $array[$elt]    undef $array[$elt]
       (or)            splice( @array, $elt, 1 )

##</code><code>##

sub all_between {
   my ($min, $max ) = @_;
   if ( $min > $max ) { return; }
   my @array = ();
   my $i = $min + 1; 
   while ($i < $max) {
      push @array, $i++;
   }
   return @array;
}


my @ar1 = all_between( 4, 10 ); # Gives (5,6,7,8,9)
my @ar2 = all_between( 4, 5 ); # Gives ()
my @ar3 = all_between( 4, 3 ); # Gives undef
print "\@ar1 is defined\n"   if  defined @ar1;
print "\@ar2 is defined\n"   if  defined @ar2;
print "\@ar3 is defined\n"   if  defined @ar3;

##</code><code>##

@ar1 is defined