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]

In reply to Re: What regex changes are needed while creating xml by toolic
in thread [Solved]:What regex changes are needed while creating xml by Perl300

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