The left side of the for-Statement is executed multiple times, so with my, $columnsum seems to become a local variable in the for-block, although use strict doesn't complain.
Try 
my $columnsum = 0; 
$columnsum += $_->{columns} for @data;
print $columnsum;

[download]
and everything seems to be fine.

In reply to Re: 'my' headache...why doesn't this work? by ls
in thread 'my' headache...why doesn't this work? by S_Shrum

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