my $value_0;
my $value_1 = 'i have a defined value';
my $value_2 = 0;

$value_0=~/content/; # this will complain
$value_1=~/content/; # won't complain
$value_2=~/content/; # won't complain

##</code><code>##

if ( defined $value_0 and $value_0=~/content/ ){
   ...