use Data::Dumper;

my $base_hash = {};

while (<DATA>) {
    chomp;
    my @keys = split ':';
    my $val = pop @keys;
    my $hashp = $base_hash;
    
    for my $key (@keys) {
        unless (exists $hashp->{$key}) {
            $hashp->{$key} = {};
        }
        $hashp = $hashp->{$key};
    }

    $hashp->{"VAL"} = $val;
}

print Dumper $base_hash;

__DATA__
global:2:3:4:LINE1
global:2:3:4:5:LINE2


########## Output ##########
$VAR1 = {
    'global' => {
        '2' => {
            '3' => {
                '4' => {
                    'VAL' => 'LINE1',
                    '5' => {
                        'VAL' => 'LINE2'
                    }
                }
            }
        }
    }
};