Semordnilap

var c = 0
var seen = Hash()

ARGF.each { |line|
    var r = line.reverse
    ((seen{r} := 0 ++) && (c++ < 5) && say "#{line} #{r}") ->
        || (seen{line} := 0 ++)
}

say c

Output:

$ sidef semordnilap.sf < unixdict.txt
ca ac
dab bad
diva avid
dna and
drab bard
158