while( my $accnt = $allAccnt->next ) {
    $accnt->calcInterest() if $accnt->inCredit();
}

##</code><code>##

sub calcInterest {
    my $self = shift;
    return unless $self->inCredit();
    ...
}

##</code><code>##

$_->calcInterest() while $allAccnts->next;