Use the -d test to see if the argument is a directory (perldoc -f -X). Use glob to get a list of all files in that directory. Repeat until you reach a file. (-d $file will be false). Then, open that file and read a line from it. Code might look something like this:

my $possible_file = shift;
 
my $found_file = find_file($possible_file);

{
    local *INPUT;
    open(INPUT, $found_file) or die "Can't open $found_file: $!";
    my $line = <INPUT>;
    # do something with $line
    close INPUT or die "Can't close $found_file.  Bogosity: $!";
}

sub find_file {
    my $file = shift;
    while (-d $file) {
        # the next line may differ on Win/Mac/Un*x
        my @files = glob("$possible_file/*.*");
        foreach (@files) {
            find_file($_);
        }
        # we're not dealing with a file anymore, so return
    }
    return $file;
}
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