How do references in Perl work

Question

Can anyone explain the push statement in the following Perl code to me? I know how push in perl works but I can\'t understand what the first argument in followi

Answer 1

Let's break it down.

The @{...} is understood from "Using References" in perlref

Anywhere you'd put an identifier (or chain of identifiers) as part of a variable or subroutine name, you can replace the identifier with a BLOCK returning a reference of the correct type.

So what is inside { ... } block had better work out to an array reference. You have $a[$b] there, an element of @a at index $b, so that element must be an arrayref.

Then @{...} dereferences it and pushes a new element $c to it. Altogether, $c is copied into a (sole) element of an anonymous array whose reference is at index $b of the array @a.

And a crucial part: as there is in fact no arrayref ^† there, the autovivification kicks in and it is created. Since there are no elements at indices preceding $b they are created as well, with value undef.

Now please work through

• tutorial perlreftut and

• data-structures cookbook perldsc

while using perlref linked in the beginning for a full reference.

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With complex data structures it is useful to be able to see them, and there are tools for that. A most often used one is the core Data::Dumper, and here is an example with Data::Dump

perl -MData::Dump=dd -wE'@ary = (1); push @{$ary[3]}, "ah"; dd \@ary'

with output

[1, undef, undef, ["ah"]]

where [] inside indicate an arrayref, with its sole element being the string ah.

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^† More precisely, an undef scalar is dereferenced and since this happens in an lvalue context the autovivification goes. Thanks to ikegami for a comment. See for instance this post with its links.