Question : perl school; undef, legit use of

I often test subs for an undef returns (return undef).  
Is it legal to test scalars   $x == undef?  It seems that
only a test against an explictly assigned value of 'undef'
is meaningful.

If not, ignor the rest of the question.


==============================================

Here a program where I test every state of $x I could think of.  Here's
the output:
  Is nothing undefined:  yes                            
  Is an unassigned declaraton undefined:  yes  # $x;
  Is an undefed var undefined:  yes                 # undef $x;
  Is a string var undefined:  yes                      # $x = "sdfsdfsdfsdf";
  Is a one var undefined:  no                          # $x = 1;
  Is var assigned to undef,  undefined:  yes      # $x = undef;
  Is var assigned to !undef,  undefined:  no      # $x = !undef;

==================================
print qq/Is nothing undefined:  /;
if ($x == undef) {
  print qq/yes\n/;
}else {
  print qq/no\n/;
}

$x;
print qq/Is an unassigned declaraton undefined:  /;
if ($x == undef) {
  print qq/yes\n/;
}else {
  print qq/no\n/;
}

undef $x;
print qq/Is an undefed var undefined:  /;
if ($x == undef) {
  print qq/yes\n/;
}else {
  print qq/no\n/;
}

$x = "this is a string";
print qq/Is a string var undefined:  /;
if ($x == undef) {
  print qq/yes\n/;
}else {
  print qq/no\n/;
}

$x = 1;
print qq/Is a one var undefined:  /;
if ($x == undef) {
  print qq/yes\n/;
}else {
  print qq/no\n/;
}

$x = undef;
print qq/Is var assigned to undef,  undefined:  /;
if ($x == undef) {
  print qq/yes\n/;
}else {
  print qq/no\n/;
}

$x = !undef;
print qq/Is var assigned to !undef,  undefined:  /;
if ($x == undef) {
  print qq/yes\n/;
}else {
  print qq/no\n/;
}

Answer : perl school; undef, legit use of

with ==, undef acts like 0
with eq, undef acts like ''
to test if a variable is defined, use
defined $x