Interesting (as long as you're not planning on using this for your next website template). I enjoy rewriting other people's code, it passes some time and I usually end up learning something new. So here's my version (not that you'd want to use this on extremely large files. I imagine this would be very inefficient as I handle the whole thing in memory):

#!c:/perl/bin/perl -w
$|++;

use strict;
use Getopt::Std;

sub show_usage;

my $text;

show_usage unless getopts('t:i:o:h', \my %opts);
show_usage if $opts{'h'} or not $opts{'t'};

# read input file
if ($opts{'i'}) {
  open my $fh, '<', $opts{'i'}
    or die "Couldn't open input file for reading: $!\n";
  $text = do { local $/; <$fh> };
  close $fh or die "Coudn't close input file: $!\n";
}

# get text from console
else {
  print "Ready for Input:\n\n";
  $text = <STDIN>;
}

my $t_open = '<' . $opts{'t'} . '>';
my $t_close = '</' . $opts{'t'} . '>';

# insert random tags
$text = join '', map {
  /\s/ || int rand 2 ? $_ : $t_open . $_ . $t_close
} split //, $text;
$text =~ s/\Q$t_close$t_open\E//g;

# output to outfile
if (defined $opts{'o'}) {
  open my $fh, '>', $opts{'o'}
    or die "Couldn't open output file for writing: $!\n";
  print $fh $text;
  close $fh or die "Couldn't close output file: $!\n";

  print "Text saved to output file $opts{'o'}.\n";
}

# make backup/save to infile
elsif (defined $opts{'i'}) {
  rename $opts{'i'}, $opts{'i'} . '.bak';

  open my $fh, '>', $opts{'i'}
    or die "Couldn't open output file for writing: $!\n";
  print $fh $text;
  close $fh or die "Couldn't close output file: $!\n";

  print "Input file $opts{'i'} backed up to $opts{'i'}.bak.\n",
    "Text saved back to output file $opts{'i'}.\n";
}

# print to console
else {
  print $text, "\n";
}

# display usage information
sub show_usage {
  print <<"END_OF_USAGE";
This script randomly inserts a specified HTML tag into some text.

Usage:
 \$ $0 -t htmltag [-i] [-o] [-h]

Options:
    -t   The html tag you wish to insert (ie: "em", "strong").
    -i   (Optional) The input file from which to read the text into
         which the random html tags will be inserted.
    -o   (Optional) The file to which the generated output will be pri
+nted.
         If you do not supply this option, the input file will be used
+ for
         output. A backup of the original input file will first be mad
+e.
         If an input file was not used, output will be printed to the 
+console.

Flags:
    -h   Displays this help message.

END_OF_USAGE

  exit;
}
[download]