./replace.pl -s '<html(.*)>' -r '<head$1>'
[download]

Thanks,

but the problem actually lies deeper in my script, for example :

$test = "hello";
$s = "(hello)";
$r = "$1 world";
$test =~ s/$s/$r/;
print $test;


output : hello and not hello world as I would expect.

Is there any way I could do this ?

Similar problem, different context. In perl, double quotes causes interpolation, so in

$r = "$1 world";
[download]

$1 is substituted with whatever was matched in a previous regex. To prevent interpolation, use single quotes.
That doesn't solve the whole problem, however. Now we need to substitute the strings, including metacharacters, into s///. To interpolate, then substitute, I'll use an eval:

$s = '(hello)';
$r = '$1 world';
$_ = 'hello';
eval "s/$s/$r/";
print $_;
[download]

A good general rule to follow is to use single quotes if you want the string as is, and use double quotes if you want interpolation.
-Mark

You're asking for it to interpolate $1 in $r when you use double-quotes. Don't do this. $1 is undef when you do that, so of course, $r ends up being " world".

Here's a really crazy example of how to do what you want.

$test = "hello";
$s = "(hello)";
$r = '"$1 world"';
$test =~ s/$s/eval $r/e;
print $test;
[download]

Note that this uses eval, which isn't everyone's thing.

Following regexes will overwrite previously set $1 and friends. Probably your best best would be to tell users that matches aren't stored in $1, $2 etcetera but in an array named @match. Then, after each user defined regex set @match to the values. This will allow them to be persistent (to a degree, based on how many regexes are fed to your script). This can be a bit confusing if there are a lot of usedefined regexes, because your users might wants a greater degree of persistance that one regex. Of course, if that happens, then you probably need a different solution that command line args.

Light a man a fire, he's warm for a day. Catch a man on fire, and he's warm for the rest of his life. - Terry Pratchet

Update: yup. perl -e"$foo = 3; print $foo;" demonstrates.

I see kvale pointed out a solution below.