0: sub seval
1: {
2:     my ($sub) = shift;
3:     $vni++;
4:     $sub =~ s#s/([^/]+)/([^/]+)/(.+)#s/\1/\$i$vni++ == \3 ? \2 : \$&/egs#;
5:     return $sub;
6: }
7: 
8: # Evals 's/foo/$i1++ == 1 ? bar : $&/egs', replacing match 3 only
9: eval(seval("s/foo/bar/2"));