$_ = '111(22(33)44)55';
my $re = qr/
    (.*?)  # minimal grab up to the first...
    \(     # literal left paren
    .*     # greedy skip everything up through...
    \)     # the last literal right paren
    (.*)$  # then grab everything remaining
    /x;
s/$re/$1$2/;
print;