my $tab = 0; ## Number of 'tabs' to insert starts at zero
$input =~ s[
    (          ## Capture to $1
       [()]    ## All open or close parens
    )
]{  ## /e-xecute turns the second half into a code block.
    ## Decrement teh tab count if this is a close paren
    $tab-- if $1 eq ')';

    ## Insert a newline + $tab tabs before the paren (open or close)
    my $modified = "\n" . ( "    " x $tab ) . $1;

    ## Increment the tab count if this is an open paren
    $tab++ if $1 eq '(';

    ## And 'return' the modified text for substitution
    $modified;
}xge;

## Late addition:
## Second pass strips out any 'lone' close parens
## to compact the results a little.
$input =~ s[\n\s+\)][)]g;