The one-liner, perl -ni -e'print; print "uid:", $1, $/ if /^cn:(.*)/;' foo.file

Update: Argh, I missed that bit, too. Off the top of my head, perl -ni -e'print; if (/^cn:(.*)/) { my $uid = "uid:$1\n"; my $nl = <>; print $nl =~ /^uid:/ ? "": $uid, $nl}' foo.file Untested.