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my @cols;
for my $row (@array) {
...
    $f[2] = $b[$f[2]];
    push @{$cols[$_]}, $f[$_] for 0 .. $#f;
}

my %col_for = (
    foo => 2,
...
    splice @f, $col_for{$f[1]} || 6, 0, shift @b;
    push @{$cols[$_]}, $f[$_] for 0 .. $#f;
}