It's not printing scalar $A->{arrayholder}), it's printing the scalar value of whatever was assigned to $A->{arrayholder}, since hash elements (incl $A->{arrayholder}) are always scalars.

If you wish to store a list in a hash element, create an array containing the list and store a reference to that array in the hash element. (References are scalars.)

$A->{arrayholder} = [ ];

@{ $A->{arrayholder} } = ( $B1, $B2 );
# -or-
#push @{ $A->{arrayholder} }, $B1, $B2;

foreach my $B (@{ $A->{arrayholder} }) {
   ...
}
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Update: Rephrased text to be a bit clearer.