#!/usr/bin/perl

use utf8;
use warnings;
use strict;

my $var1;
my $var2 = "0";
my $var3 = 0;
my $var4 = "a";

print isDef($var1)."\n";
print isDef($var2)."\n";
print isDef($var3)."\n";
print isDef($var4)."\n";

sub isDef {
  if (shift) {
    return "true";
  } else {
    return "false";
  }
}