Because of your 'use strict' pragma; try this instead:

for my $n (@nm1)
{
    print "\n$n:";
    no strict 'refs';
    for my $m (keys %{$n})
    {
        printf "\t%3s: %-8s", $m, ${$n}{$m};
    }
}
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and then take a look at perldoc perlref (search for strict).