1. I don't know of any way to cause a hash key to be created by use of an "if" test.

2. I don't know of any way to cause a hash key's value to be "non-existant". There is a thing called "undefined", undef. But undef is not the same as "non-existant". undef means exists but I don't know what the value is.

3. When you test a hash key, you are testing the value of the key. It can be true or false. false values are: "undef","",'',0.

4. If a hash key value "exists" then it can have any one of the 4 values above. Update: well of course, then it can also have some other string or numeric value. The above 4 things, which are actually only 3 things, undef, null string and zero are all the same "false" value in a logical expression.

5. If a hash key value is "defined", then there only 3 possibilities. Update: well "" and '' are the same once the string is interpreted.

#!/usr/bin/perl -w
use strict;

use Data::Dumper;

my %hash =  ('a' => 2,  'b' => 3, 'c' => undef);
print Dumper (\%hash);

if (exists ($hash{'c'}) )
  {print "key c exists\n"}
else
  {print "key c does not exist\n"};
print Dumper (\%hash);

if ( defined ($hash{'c'}) )
  {print "key c  defined\n"}
else 
  {print "key c not defined\n"};
  
if (exists ($hash{'d'}) )
  {print "key d exists\n"}
else
  {print "key d does not exist\n"};

#note that undef,"",'' and 0 all evaluate to "false"
#play with c value above and run this code
#you can call defined($xxx) to figure out the difference
#between a false value from "",'',0 and undef.

if (my $x = $hash{'c'}) 
   {print "c has value $x\n"}
else
   {print "c has value,\"\",0 or undef\n"};

if (my $x = $hash{'b'}) 
   {print "b has value $x\n"}
else
   {print "b has no value\n"};
__END__
$VAR1 = {
          'c' => undef,
          'a' => 2,
          'b' => 3
        };
key c exists
$VAR1 = {
          'c' => undef,
          'a' => 2,
          'b' => 3
        };
key c not defined
key d does not exist
c has no value
b has value 3
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