Thank you all for your suggestions. Finally I used the following code to solve the problem:

sub function1
{
my $var = defined $_[0] ? shift : 'No argument';
print "$var\n";
}
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But I still wonder if we cannot assign string value "0" to a variable using short circuit.

// works well, but that is in Perl 5.10.0. What if we have a situation like the following (in Perl 5.8.8), where we have multiple expressions involved in the short circuit in order of preference:

my $var = $hash1{$name} || $hash2{$name} || $some_default_value;
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This will assign the default value to $var if say the key $name is not defined in %hash1 and is assigned a value 0 in %hash2. Is a if-elsif ladder or complex nested ternary operators the only way to solve this? Or may be this is the reason why they came up with // in Perl 5.10.0