I'd do this:

$statistics{$line_number} ||= [];
push @{ $statistics{$line_number} }, $thisline;
[download]

If $statistics{$line_number} is already an array ref, it is considered true, so the ||= operator won't affect it. Otherwise, it becomes an empty array ref.

Whatever happens on the first line, you know on the second line that $statistics{$line_number} is an array ref, so you can push without further pondering.

HTH

--bwana147