I got rid of $string1 since you don't seem to use it, and I changed the 1st line of the regex:

use warnings;
use strict;

while (<DATA>) {
    chomp;
    my ($string2, $subscript_name, $subscript_value) = /
        ^[^:]+::
        ([^\s]+)
        \.([^\s]+)
        \s(.*)
        /x;
    print "$string2\n";
}

__DATA__
GI-eSTB-MIB-NPH::eSTBUserSettingOutput.0 = INTEGER: hd1920x1080i(1)
GI-eSTB-MIB-NPH::eSTBUserSetting43OverRide.0 = INTEGER: on480p(3)
[download]

Outputs:

eSTBUserSettingOutput
eSTBUserSetting43OverRide
[download]