Again, your data is something else than you've showed us.

#!/usr/bin/perl
use warnings;
use strict;

my $xst = {
    'cmts' => {
        'STBDSG' => {
            'dsg' => [
                     {
                       'tag1' => '1',
                       'tag2' => 'caSystemId',
                     },
                     {
                       'tag1' => '2',
                       'tag2' => 'gaSystemId',
                     }
                     ]
                    }}};

my %dsg_tags;
$dsg_tags{'tag1'} = $xst->{cmts}{STBDSG}{dsg}[0]{tag1};

foreach (sort keys %dsg_tags) {
    print "$_ : $dsg_tags{$_}\n";
}

__END__
Output:
tag1 : 1
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