Here is my contribution

sub get_inner {
   my $str   = shift;
   my $inner = ( split(/\(/, $str) )[-1];
   $inner    =~ s/\).+$//;

   $inner;
}
[download]

Im sure it could be golfed, but *shrug* HTH
Update: or the reverse would be

sub get_inner {
   my $str   = shift;
   my $inner = ( split(/\)/, $str) )[0];
   $inner    =~ m/([^\(]+)$/;

   $1;
}
[download]

MMMMM... Chocolaty Perl Goodness.....