#! /usr/local/bin/perl5.9.5

use strict;
use warnings;

a(10);
a(0);
a(20);

sub a {
    my $num = shift;
    my $x = $num * 100 if $num;
    print $x, $/;
}
# produces
# 1000
# Use of uninitialized value $x in print at ./myif line 13.
# 2000