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

##</code><code>##

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

##</code><code>##

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