use IO::String;
my $regex = qr/sample/;
my $file = 'file.html';

open my $in , '<', $file or die "Can't open $file: $!\n";
my $new_html;
my $new = IO::String->new($new_html);

my $last = '';
my $modified;
while ( my $line = <$in> ) {
    if ( $line =~ /$regex/ ) {
        if ($line ne $last) {
            print {$new} $line;
        }
        else {
            ++$modified;
        }
    }
    else {
        print {$new} $line;
    }
    $last = $line;
}

close $in;
close $new;

if ($modified) {
    rename $file, "$file.bak" or die "Can't rename $file: $!\n";
    open my $out, '>', $file or die "Can't open $file: $!\n.";
    print {$out} $new_html or die "Can't print to $file: $!\n."
}