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#!/usr/bin/perl

...
  perl tricky_html_filter.pl foo.html

=cut

my $file = shift || 'E:/path/to/dummy1.html';
open INFILE, '<', $file, or die "$!: can't open $file\n";

while (<INFILE>) {
    s/(background-color:#)(?:[0-9a-f]{6}|[0-9a-f]{3})/$1ffffff/i;
    print;
}

while (<INFILE>) {
    print;
}

while ($_ = <INFILE>} {
    print $_;
}

while (my $line = <INFILE>} {
    print $line;
}

s/(background-color:#)(?:[0-9a-f]{6}|[0-9a-f]{3})/$1ffffff/i;

$1ffffff

#!/usr/bin/perl

...
    s/(background-color:#)(?:[0-9a-f]{6}|[0-9a-f]{3})/$1ffffff/i;
    print;
}

perl -pi -e"s/(background-color:#)(?:[0-9a-f]{6}|[0-9a-f]{3})/$1ffffff
+/i" /path/to/dummy1.html

# be sure and replace " with ' if you run this on *NIX