my $fn="files.txt"; 
open OUT,">$fn" || die $!; 
print OUT grep(!/^$fn$/,map { $_.="\n"; $_ } <*>); 
close OUT; 
 
##</code><code>##

my $dir="thedir"; 
my $fn="files.txt"; 
open OUT,">$dir/$fn" || die $!; 
print OUT grep(!/^$fn$/,map { s/^$dir\///; $_.="\n"; $_ } <$dir/*>); 
close OUT;