You mean, instead of deleting tags, you want to replace them with strings of spaces, so that the output file is the same size as the input, and just has a lot more space characters (and no html tags) -- have I got that right?

This would do:

#!/usr/bin/perl

use strict;
use HTML::TokeParser::Simple;

my $htm = HTML::TokeParser::Simple->new( $ARGV[0] ) or die "oops: $!";

while ( my $token = $htm->get_token )
{
    if ( $token->is_tag() ) {
        print " " x length( $token->as_is );
    } else {
        print $token->as_is;
    }
}
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If you look at the perldoc man page for HTML::TokeParser::Simple (and the "less simple" classes it is derived from), you might find it easy to come up with other more useful variants, and/or figure out handy ways to deal with things like scripting and comments that are often included in html files.