#!perl
use strict;
use warnings;

my $dir = 'some/directory';
opendir( my $d, $dir ) or die "can't open dir $dir: $!\n";
open my $o, '>', './output' or die "can't write to file: $!\n";
print {$o}
    map { open my $in, '<', sprintf '%s/%s', $dir, $_; local $/; <$in> }
    grep { -f sprintf '%s/%s', $dir, $_ }
    readdir $d;
close $o or die "can't complete writing to file: $!\n";
closedir $d;