Nowadays, if I have to do anything with files I use Path::Tiny, it makes code much cleaner. Here's how your script would look with it:

#!/usr/bin/env perl
use strict;
use warnings; 
use Path::Tiny;

my $dir     = Path::Tiny->cwd;
my @subdirs = grep { $_->is_dir } $dir->children;

my $results = $dir->child('results');

for my $subdir (@subdirs) {
    my $ressubdir = $results->child($subdir->basename);
    $ressubdir->mkpath;
    chdir $subdir;
    system("pwd > $ressubdir/outfile.txt");
}

chdir $dir;
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