use strict;
use warnings;

my $stylesheet_file = "stylesheet.txt";

sub read_stylesheet
{
     open(STYLESHEET, "<", $stylesheet_file)
        || die "Cannot read stylesheet_file\n";

     my (%tag, $header);
     while (<STYLESHEET>)
     {
          if (/^HEADER\s+(\d+):/)
          {
               $header = $1;
          }
          elsif (/^nm\s+:\s+(\w+)/
          {
               # Make a note of what header this tag
               # appeared in.
               $tag{$1} = $header;
          }
     }
     close(STYLESHEET);

     return \%tag;    # Returns a reference to the hash
}

# ... (Main routine)

my $input_file    = "taggedfile.txt";
my ($output_file) = @ARGV;

my $tag = read_stylesheet();

open(FILE, "<", $input_file)
     || die "Could not read $input_file\n";
open(OUT, ">", $output_file)
     || die "Could not write $output_file\n";

while (<FILE>)
{
     # Perform substitutions
     s/^\{(\w+)\}/\{$1:$tag->{$1}\}/g;

     # Write line ($_) to OUT
     print OUT;
}

close(FILE);
close(OUT);

##</code><code>##

while (my $line = <FILE>)
{
     # ... Use $line where you would normally use $_
}