To reuse getCreditAccounts(), you'd have to first rewrite the method to return array ref, and not a copy of the array you are attempting to ultimately change.

# private getCreditAccounts
sub getCreditAccounts {
    caller(0) eq __PACKAGE__ || confess "getCreditAccounts is private"
+;
    my $closure = shift;
    my @accounts = @{ &{$closure}( "CREDITACCOUNTS" ) };
    return @accounts;
}

# working on a copy of the original array...
my @anotherArray = $obj->getCreditAccounts();
push @anotherArray, "Data";
[download]

From there, you could possibly do something like

# private getCreditAccounts
# now returning reference to the original array
sub getCreditAccounts {
    caller(0) eq __PACKAGE__ || confess "getCreditAccounts is private"
+;
    my $closure = shift;
    return &{$closure}( "CREDITACCOUNTS" ) };
}

# public to add accounts to the creditaccounts
sub addCreditAccount {
    my $closure  = shift;
    my $account = shift;
    push @{ $closure->getCreditAccounts() }, $account;
}
[download]