In your case you would have to extract the single diskstats into an array again :

build_histogram ($bin_stage, map $_->{'stagetime'},@diskxStats );
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This is equivalent to the following code which might or might not be easier to understand:

my @statetime=();
foreach (@diskxStats) {
  push @statetime, $_->{'stagetime'};
}
build_histogram ($bin_stage, @statetime );
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