Hello winterwind, and welcome to the Monastery!

Re-read davido’s answer, he has explained the use of ++. Although a literal undef++ is a syntax error, that is essentially what is happening here:

12:11 >perl -wE "++$x; say $x;"
1

12:12 >perl -Mstrict -wE "my $x; ++$x; say $x;"
1

12:13 >
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This is Perl, it’s a feature! For more on autovivification, see the 2008 thread Explaining Autovivication, and especially Uri Guttman's Tutorial.

Hope that helps,

You've pretty much got it. In Perl, all scalar containers (ie, a scalar variable, an element from an array, or an element from a hash) start out as "undef", which is different from C's notion of undefined. In C, undefined means "could be anything, but never anything useful". In Perl, undefined means "undef", the value. In numeric context, undef equates to zero, meaning if you treat undef as a number it acts like zero. Increment zero, and you get 1.

You can explicitly detect definedness with defined:

perl -E 'say "undefined" if ! defined($var);'
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...or...

perl -E '$var = 1; say "defined" if defined($var);'
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But numeric operations, including Boolean comparisons in numeric context cause undef to act like a "false" numeric value, which is to say, zero.

perl -E 'say "Hello" if undef == 0;'
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...or...

my $var;  # Currently $var is undef
print $var + 5, "\n"; # 5
print ++$var, "\n";   # 1
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That's really the less amazing thing. The more amazing thing is that you can implicitly create references to anonymous data structures. Consider this:

my %hash;
$hash{foo} = { bar => 'baz' };
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The { ... } curly braces around  bar => 'baz' act as explicit anonymous hash constructors, so the resulting structure looks like this:

%hash = (
  foo => {        # { is an anon-hash constructor
    bar => 'baz'
  }
);
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That's the boring explicit way. But this will create the exact same structure, without explicitly constructing the inner hashref:

my %hash;
$hash{foo}{bar} = 'baz';
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The { bar => 'baz'} hash ref was constructed implicitly.