A small point further to the excellent replies of Corion and gmargo:

While the OP defines the state of the  %labels hash, it does not say if  $tablename is initialized or not; the signs and symptoms will differ in the two cases (assuming warnings and strictures are used – as they always should be!).

If  $tablename is undefined, it will be 'coerced' to the empty string, which is a valid hash key. 

>perl -wMstrict -le
"my %labels;
 my $tablename;
 $_ = 'foo';
 push @{ $labels{$tablename} }, $_;
 use Data::Dumper;
 print Dumper \%labels;
"
Use of uninitialized value $tablename in hash element at ....
$VAR1 = {
          '' => [
                  'foo'
                ]
        };

>perl -wMstrict -le
"my %labels;
 my $tablename = 'bar';
 $_ = 'foo';
 push @{ $labels{$tablename} }, $_;
 use Data::Dumper;
 print Dumper \%labels;
"
$VAR1 = {
          'bar' => [
                     'foo'
                   ]
        };

[download]

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

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.