What am I doing wrong?

Enable warnings (Tip #1 from Basic debugging checklist). This will show that you are using the wrong equality operator. I have added code to make yours runnable (and used perltidy):

use warnings;
#use strict;
use Getopt::Long;

$result = GetOptions( "OIMID=s" => \$oimid, );

sub StingtoNum() {
    if ( $oimid eq "Google" ) {

        return "01";
    }
    elsif ( $oimid eq "DELL" )

    {
        return "02";
    }
    else {
        return "00";
    }
}

sub Add() {
    $oim     = &StingtoNum;
    $version = "00";
    $sum     = $oim . $version;
    print "$sum";
}

Add();
print "\n";
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