my $dir = shift; # get the first command line arg
my $path = "/remote/scm/rsync/builds/$arg/FailedInformal";

opendir DIR, $path or die "Can't open $path: $!\n";
my @releases = grep {/^\d/} readdir DIR;

# NOW, it's up to you what you want with files you find
# in @releases

# get the latest three
my @three = @releases[-1,-2,-3];

# get only the very latest
my $latest = $relreases[-1];
....

# and closing the DIR handle is good
closedir DIR;

##</code><code>##

open FILE, $latest or die "Can't open $latest: $!\n";
....

##</code><code>##

# presume / as path separator
my $latest_file = "$path/$latest";
open FILE, $latest_file or die ...;