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'
                   ]
        };
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