my $indir = "/tmp/a";
my $outdir = "/tmp/b";
my @suffix = qw(A B C D E F ); ## ON AND ON

foreach my $sfilename (@infiles) {
    $ofilename = "$outdir/$sfilename";
    if (-e $ofilename) {
        ## it exists, lets add a suffix
        for (@suffix) {
            if (! -e "$ofilename.$_") {
                rename "$indir/$sfilename","$ofilename.$_";
                print "Renamed to $ofilename.$_";
                exit(0);
            } else {
                ## suffix alreay taken, try next
                print "$ofilename.$_ exists ...\n";
            }
        }
    } else {
        ## does not exist, rename
        rename "$indir/$sfilename", "$ofilename";
    }
}