@a = 'bar';
@bar = 6;

warn 'PUSHING ONTO @BAR' if defined $a[0];

push @{$a[0]}, 'bar';

undef $a[0];

warn 'PUSHING ONTO $a[0]' if not defined $a[0];

push @{$a[0]}, 'bar';

use Data::Dumper;
warn Dumper( \@a, \@bar );

__END__
PUSHING ONTO @BAR at - line 4.
PUSHING ONTO $a[0] at - line 10.
$VAR1 = [
          [
            'bar'
          ]
        ];
$VAR2 = [
          6,
          'bar'
        ];