my $var1; # not printed because $var1 is not defined print "var1: $var1\n" if defined $var1; my $var2 = ''; # printed with $var2 as an empty string # output var2: print "var2: $var2\n" if defined $var2; my $var3 = 0; # printed with $var3 set to 0 # output var3: 0 print "var3: $var3\n" if defined $var3;