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Very interesting!

At first glance your code didn't look right. So I read more carefully and saw what was bothering me:

push @{$dataPos{$data[$_]}}, $_ for 0 .. $#data;

You are pushing data onto an undefined value that is dereferenced as an array! The script should (I thought) die the first time through the for loop. But it works. So I did some experimentats and read the docs.

I found out that you can do this:

my $foo; # $foo is undefined push @$foo, 23; print "I found @{$foo}\n";

Further testing showed that:

push @{ undef() }, 99;
Died as I had expected.

Further tests show that unshift works the same way. Also, pop and shift will create array references.

my $foo = undef; shift @{$foo}; print "Foo: $foo\n"; # prints Foo: ARRAY(0x12121212)

The various docs don't mention this special behavior. Thanks for the education.

TGI says moo

In reply to Re^2: hash and array mismatch by TGI
in thread hash and array mismatch by dana

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