What's happening is "autovivification" of an array reference.

A quote from the book:
Programming Perl, 3rd Ed., p.251
Chapter 8: References
"Implicit Creation of References"

References of an appropriate type simply spring into existence if you dereference them in an lvalue context that assumes they exist.

So your statement: 

    push @{$labels{$tablename}}, $_;

[download]
is the same as: 
    $labels{$tablename} = [] if !exists $labels{$tablename};
    push @{$labels{$tablename}}, $_;

[download]
See also autovivification in the glossary, and perlref.

In reply to Re: Figuring out some data structure by gmargo
in thread Figuring out some data structure by alfredjunkyardgreen

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