$name = ..... # whatever
my $i= '';
--$i while (-e $name . $i);
$name.= $i;

##</code><code>##

my $check= $name;
$check=~ s#(\.[^./]*)$##;
my $ext= $1 || '';
my $i= '';
--$i while (-e $check . $i . $ext);
$name= $check . $i . $ext;