my $var;     # variable is undefined
if ($var) { ... }  # $var is false because it is undefined
if (defined $var) {...}  # false

$var = "";
if ($var) { ... } # false because $var is null string
if (defined $var) {...} # true because $var has been
                        # defined as having a null string

$var = 0;
if ($var) { ... } # false because $var is zero
if (defined $var) {...} # true because $var has been
                        # defined as zero