...
    for (;;) {

        // --- add this
        printf("\n");     // the prompt
        fflush(stdout);   // make sure output gets written
        // ---

        if (fi) {
            if (!fgets(slowo,32,fi)) break;
    ...

##</code><code>##

...
sub read_sap_response {
    while (my $line = <SAPR>) {
        last if $line =~ /^$/;  # found "prompt" --> exit loop
        print $line;            # show response
    }
}

my $pid = fork;
if ($pid) {
    close PARENTR; close PARENTW;

    read_sap_response();
    # this is needed, because we've patched the program to output
    # the blank line _before_ waiting for input (which was the
    # easiest option in this case...)

    while (1) {
        open FIFOR, "<fifo";
        my $line = <FIFOR>;
        close FIFOR;
        print SAPW $line;
        read_sap_response();
    }
} else {
    close SAPR; close SAPW;

    open STDIN , '<&PARENTR';
    open STDOUT, '>&PARENTW';

    exec "sap";
    die $!;
}