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;