$ perl -MO=Deparse -e 'my $columnsum += $_->[$colnum] for @data;'
foreach $_ (@data) {
    my $columnsum += $$_[$colnum];
}
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The my declaration is within the for loop, so for each @data, a new $columnsum is created.

Update: As demerphq has pointed out, the deparse output is wrong for this snippet. The todo section at the top of B::Deparse even mentions this specifically. Now I'm off to search for what it is really doing...

Update2: A google search for "my $x if 0" returns some answers. There is even a node here in the monestary that talks about it.