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  1. or download this
        perl -ne 'print $prev, $_ if /foobar/; $prev = $_' file
  2. or download this
        perl -ne 'BEGIN{$re=qr/foo/}if(/$re/){print$p if$p!~/$re/;print$_}
+;$p=$_' file
  3. or download this
        grep -B 1 foobar file
  4. or download this
        perl -ne '$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