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my $csv = Text::CSV_XS->new ({ binary => 1 });
open my $dta, "<", "data.csv" or die "data: $!\n";
my @rows;
...

# pivot
my @data = map { my $col = $_; join "|", map { $_->[$col] } @rows } 0.
+.$#{$rows[0]};

123,foo,2,FOO
234,bar,2,BAR
345,zap,3,ZAP
456,bok,5,BOK

(   "123|234|345|456",
    "foo|bar|zap|bok",
    "2|2|3|5",
    "FOO|BAR|ZAP|BOK")