perl -ne 'print $prev, $_ if /foobar/; $prev = $_' file

##</code><code>##

    perl -ne 'BEGIN{$re=qr/foo/}if(/$re/){print$p if$p!~/$re/;print$_};$p=$_' file

##</code><code>##

    grep -B 1 foobar file

##</code><code>##

    perl -ne '$i=$.% 2; $a[$i]=$_; print @a[$i+1..$#a,0..$i] if /foo/' file
    perl -ne '$i=$.% 2; $a[$i]=$_; print @a[$i+1..$#a,0..$i] and @a=() if /foo/' file