Question

use strict;
my $var = NULL;

will raise an error of Bareword "NULL" not allowed while "strict subs" in use

Answer 1

There’s no NULL in Perl. However, variables can be undefined, which means that they have no value set.
Here’re some examples of how you can get an undefined variable in Perl:

my $var;       # variables are undefined by default
undef $var;    # undef() undefines the value of a variable
$var = undef;  # same, using an alternative syntax

To check for definedness of a variable, use defined(), i.e.

print "\$var is undefined\n" unless defined $var;