#!/usr/bin/perl

use strict;
use warnings;

my @files = qw{file1.txt file2.txt};

my %paragraphs;

for my $file (@files){
  open my $fh, q{<}, $file
    or die qq{cant open *$file* to read: $!\n};
  local $/ = q{};
  while (my $para = <$fh>){
    chomp $para;
    push @{$paragraphs{$file}}, sprintf(q{<p>%s</p>}, $para);
  }
}

for my $file (sort keys %paragraphs){
  printf qq{\n*** %s ***\n\n}, $file;
  for my $para (@{$paragraphs{$file}}){
    print qq{$para\n\n};
  }
}