my $bpt_counter=0;
my $ept_counter=0;

for ($YOUR_ENTIRE_TEXT_IN_ONE_STRING) {
  s/(&lt;p&gt;)/++$bpt_counter;qq(<bpt i="$bpt_counter" x="$bpt_counter">$1</bpt>)/eg;
  s|(&lt;/p&gt;)|++$ept_counter;qq(<ept i="$ept_counter">$1</ept>)|eg;
}

#just to be safe
warn "Uh-oh!\n" unless $bpt_counter == $ept_counter;