#!/usr/bin/perl
use strict;
use warnings;

# file given as command line argument
my $file = shift @ARGV;
die "Usage: $0 <file.xml>\n" unless $file;

# step 1: open the file
open FH, "<", $file or die "Can't open '$file': $!\n";

# step 2: read the file line-by-line
while (<FH>) {
  # step 3: do the replacements
  s/<(\/?)H([12])>/<$1Heading $2>/g;
  # step 4: write the results the STDOUT
  print;
}

close FH;