use warnings;
use strict;

my $str1 = "THIS IS OUTSIDE (THIS IS INSIDE)";
(my $x = $str1) =~ s/\(.*?\)//g;
# $x is "THIS IS OUTSIDE "

my $str2 = "OUTSIDE (INSIDE) OUTSIDE";
(my $y = $str2) =~ s/\(.*?\)//g;
# $y is "OUTSIDE  OUTSIDE"

my $str3 = "OUT(IN)OUT(IN)OUT(IN)(IN)OUT";
(my $z = $str3) =~ s/\(.*?\)//g;
# $z is "OUTOUTOUTOUT"