Earlier I recommended

perl -ne'if (s/.../.../) { print } else { print STDERR }'
    <access_log 2>out.err >out.csv
[download]

What I had in mind was

perl -ne'print { s/.../.../ ? STDOUT : STDERR } $_'
    <access_log 2>out.err >out.csv
[download]

It just occured to me that it can be shortened to

perl -pe'select s/.../.../ ? STDOUT : STDERR'
    <access_log 2>out.err >out.csv
[download]