You don't have to check for existence. You can just use an undefined value as an arrayref and it will autovivify: 
    foreach my $ItemID (@Data){ 
        push @{$$HashRef{$ItemID}}, $DataSource;
# The usual preferred notation is
#       push @{$HashRef->{$ItemID}}, $DataSource;
    }

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Caution: Contents may have been coded under pressure.

In reply to Re: Complex Data Structure Suggestions Wanted by Roy Johnson
in thread Complex Data Structure Suggestions Wanted by Ninthwave

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