Or, in a way that reads more naturally (to me, at least)

my $inc; 
$inc++ until ( not -e "$filename~$inc" );
[download]

Note that starting with an undefined $inc eliminates the need for the first check. In other words, the whole sub would be as easy as:

sub good_filename {
    my $filename = shift;
    my $inc; 
    $inc++ until ( not -e "$filename~$inc" );
    return "$filename~$inc";
}
[download]

-sauoq
"My two cents aren't worth a dime.";