while ( my @lines = $it->() ) {
    next if ($. == 1..88);
    my $letter = 'A';
    my $acctNum = do { $lines[0] =~ /\s+(\d+)\s+(\d+)/; $1 . $2 };
    push @lines, " acctnum=$acctNum";

    print for map { s/\s+/$letter++ . ' '/e; $_ } @lines;
    print "\n";
}