my $refsize = (-s $filename);
my $appendstr = "12345678\015\012";

if ( open( FH, ">>$filename" )) {
    print FH $appendstr;
    close FH;
    if (-s $filename != $refsize + length( $appendstr ) {
        warn "open succeeded on $filename, but write failed";
        # something bad
    } else {
        # something good
    }
} else {
    # a different something bad
}