It was untested code so I messed up the brackets to the right of '@'. It needs to be curly braces '{}' in that case.

    foreach( @{$result->MetricAlarms} ){

        print "-"x79,"\n";
        print "**AlarmName** : ", $_->AlarmName,"\n";
        print Dumper($_);
    }
    print "\n***\n--number of elements: ", scalar @{$result->MetricAla
+rms}, "\n";
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Alternatively you can use an intermediate variable to hold the array ref before de-referencing it.

my $ar_ref = $result->MetricAlarms;
foreach( @$ar_ref ){
...
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This article shows how to use the curly braces for dereferencing things. Also have a look at this node and this article in the Perl docs.