If you want to increment the value as each substitution is made, consider using the /e modifier for your substitution regex, which evals the replacement text before making the substitution. Something like this might work, but only if you have a matching number of opening and closing tags, and only if none of them are nested

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_counte
+r">$1</bpt>)/eg;
  s|(&lt;/p&gt;)|++$ept_counter;qq(<ept i="$ept_counter">$1</ept>)|eg;
}

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