my $path = "E:/Documents and Settings/Richard Lamb/My Documents/HTML";

open INFILE, "$path/dummy.html"
  or die "$!: Can't open this file";
open OUTFILE, ">$path/new_dummy.html"
  or die "$!: Can't write to the HTML file";

while (<INFILE>)
{
    s/(<h[1-6]\s+style=.*)">/$1; word-spacing: 50px">/ig;
    s/(<li\sstyle=.*)">/$1; word-spacing: 20px">/ig;
    s/(<p style=.*)">/$1; word-spacing: 20px">/ig;
    print OUTFILE $_;
}

close INFILE;
close OUTFILE or die "$0: $!";

rename "$path/new_dummy.html", "$path/dummy.html"
  or die "$0: $!";