c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
;;
my $s = qq{www 3 \n999\n14 xx\n}
. qq{998\nyyy 159z\n997\n}
;
print qq{[[$s]]};
;;
{ my $n = 1;
$s =~ s{ <(foo)> \s* \K \d+ (?= \s* \g-1>) }{ $n++ }xmsge;
}
print qq{[[$s]]};
"
[[www 3
999
14 xx
998
yyy 159z
997
]]
[[www 1
999
2 xx
998
yyy 3z
997
]]