'Twas brillig, and the slithy toves did gyre and gimble in the wabe

It gives you a warning if you use warnings — otherwise that block silently goes on the else path. undef is actually fairly close to a NULL style value, can be used as that fairly easily. But there is no difference between having an undef value and not existing in the first place, unless you use strict (you should) which will gladly smack your fingernails if you use an undeclared variable. It should be noted that any and every variable declared will initially have an undef value.

ralesk@eretnek:~$ perl -e 'use warnings; undef $foo; if ($foo == 2) { print "do something\n"; }'
Use of uninitialized value $foo in numeric eq (==) at -e line 1.
ralesk@eretnek:~$ perl -e 'use strict; undef $foo; if ($foo == 2) { print "do something\n"; }'
Global symbol "$foo" requires explicit package name at -e line 1.
Global symbol "$foo" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.
ralesk@eretnek:~$ perl -e 'use strict; my $foo; undef $foo; if ($foo == 2) { print "do something\n"; }'
ralesk@eretnek:~$