$text = "<paragraph>some <paragraph>some data</paragraph>data</paragraph><paragraph>more data</paragraph><paragraph>even more data</paragraph>";

my $depth = 0;
$text =~ s{(<(/)?paragraph>)}{check($depth)}gie;

print "$text\n";

sub check {
    if($2) {
        $_[0]--;
        if($_[0] == 0) {
            return $1;
        }
        $_[0] = 0 if $_[0] < 0;
    } else {
        $_[0]++;
        return $1 if $_[0] == 1;
    }
    return '';
}