johngg@aleatico:~/perl/Monks$ perl -Mstrict -Mwarnings -E 'say q{};
my $text = <<__EOT__;
cat
dog
cat
mouse
eel
cat
housecat
catamaran
fish
__EOT__

my $n = 0;
$text =~ s{cat(?{ $n++ })}{$n}g;
say $text;'

1
dog
2
mouse
eel
3
house4
5amaran
fish