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defined %hash          scalar keys %hash

defined @array         scalar @array
...

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

sub all_between {
   my ($min, $max ) = @_;
   if ( $min > $max ) { return; }
...
print "\@ar1 is defined\n"   if  defined @ar1;
print "\@ar2 is defined\n"   if  defined @ar2;
print "\@ar3 is defined\n"   if  defined @ar3;