If $alice and $bob were not assigned values, they are undef. Unlike a language such as C where using a variable that has never been assigned can result in undefined behavior, in Perl the behavior is predictable, and documented. undef is treated like an out-of-band value; it's neither a string nor a number, it's just undef. The undef value has several features:

• Its presence can be tested or using defined, //, and //= (perlop).
• If it is used somewhere that Perl expects to find a string, it will be upgraded for the duration of that operation to being an empty string.
• If it is used somewhere that Perl expects to find a number, it will be upgraded for the duration of that operation to being equal to zero.
• If it is used in Boolean context it is treated as false.
• Numeric and string upgrades will trigger a warning in most cases if the warnings pragma is in effect.
• These upgrades slightly modify the internal structure of what Perl stores in the container we know as a scalar. These modifications should not be of interest in "everyday" work, but can be observed with the Devel::Peek module.

Here are some examples:

use strict;
use warnings;
use Devel::Peek;

{
    my $foo;
    print '$foo is ', (defined($foo) ? 'defined' : 'undef'), "\n";
    Dump($foo);
}
print "\n\n";


{
    my $foo;
    print '$foo eq $foo: $foo is stringwise ', ($foo eq $foo ? 'equal'
+ : 'unequal'), "\n";
    Dump($foo);
}
print "\n\n";

{
    my $foo;
    print '$foo == $foo: $foo is numerically ', ($foo == $foo ? 'equal
+' : 'unequal'), "\n";
    Dump($foo);
}
[download]

The output will look like this:

$foo is undef
SV = NULL(0x0) at 0x1275f40
  REFCNT = 1
  FLAGS = ()


Use of uninitialized value $foo in string eq at mytest.pl line 17.
Use of uninitialized value $foo in string eq at mytest.pl line 17.
$foo eq $foo: $foo is stringwise equal
SV = PV(0x1252e70) at 0x12a0470
  REFCNT = 1
  FLAGS = ()
  PV = 0


Use of uninitialized value $foo in numeric eq (==) at mytest.pl line 2
+4.
Use of uninitialized value $foo in numeric eq (==) at mytest.pl line 2
+4.
$foo == $foo: $foo is numerically equal
SV = PVNV(0x12510f0) at 0x12a0608
  REFCNT = 1
  FLAGS = ()
  IV = 0
  NV = 0
  PV = 0
[download]

The first block tests for definedness, and then shows what's inside the scalar. Essentially it's empty, just contains undef.

The second block tests the scalar for string equality. This will cause a PV (string value) to be added to the scalar's internal structure, but the PV is =; it's empty. A future test for definedness would still be true. Perl happily just treated undef values as empty strings and compared them. Notice that warnings were generated.

The third block tests the scalar for numeric equality. This has caused an IV and NV (integer and numeric value) field to show up in the scalar struct. They are also empty. The PV field showed up again too, and is also empty. The scalar will still report to be undef if tested with defined. But internally its structure has been altered a little. Notice the warnings again, for doing a numeric comparison on uninitialized value.

All this is hidden in the internals. Mostly you just have to keep in mind that if it hasn't been initialized it will be undef, and that Perl is willing to let you do string and numeric operations and comparisons on undef, upgrading to the appropriate datatype as needed.

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