use strict;
use warnings;

my $scalar; # starts empty ie undef

                                                   # IF        THEN      ELSE
print "\$scalar now holds the following value: [", $scalar ? $scalar : 'UNDEF',"]\n";


# oops more correct as afoken pointed below:

print "\$scalar now holds the following value: [", defined $scalar ? $scalar : 'UNDEF',"]\n";

# update:
# infact, unless you check with 'defined' a zero can mislead you:
$scalar = 0;
print "\$scalar now holds the following value: [",  $scalar ? $scalar : 'UNDEF',"]\n";
print "\$scalar now holds the following value: [", defined $scalar ? $scalar : 'UNDEF',"]\n";