The  cwd() function gives the full path of the... well, CWD. The match looks for a file at the end (rightmost part) of the path. If the file is not found, the regex engine effectively backtracks up a level in the directory.

The  ! -f "$1/.marker" code in the  (??{ CODE }) construct evaluates to  '' (empty string) if the file exists at a given level, and this is interpolated into the regex pattern and always matches, hence the file is "found" at that level.

If the file is not found, the negated file test evaluates to  '1' and this will probably not be found at the beginning of a directory name. However, this can fail (debug print added to show progress):

use strict;
use warnings;
use Cwd;

my $marker = '.marker';

cwd =~ m#^(.*)(?:/|$)(??{ print "at level '$1' \n";  ! -f "$1/$marker"
+})# or die "'$marker' not found";
print "found '$marker' in '$1' \n";
[download]

Output (with no  .marker file present anywhere):

c:\@Work\Perl\monks\1nickt\two\one\zero>perl find_marker_1.pl
at level 'c:/@Work/Perl/monks/1nickt/two/one/zero'
at level 'c:/@Work/Perl/monks/1nickt/two/one'
at level 'c:/@Work/Perl/monks/1nickt/two'
at level 'c:/@Work/Perl/monks/1nickt'
at level 'c:/@Work/Perl/monks'
found '.marker' in 'c:/@Work/Perl/monks'
[download]

If you have Perl version 5.10+, there's a variation that avoids this problem:

use 5.010;  # needs (?(?{ CODE })yes-pattern) regex extension

use strict;
use warnings;

use Cwd;

my $marker = '.marker';

cwd =~ m{
    \A (.*) (?: / | \z)
    # (??{ print "at level '$1' \n";  ! -f "$1/$marker"})
    (?(?{ print "at level '$1' \n";  ! -f "$1/$marker" }) (*FAIL))
    }xms
    or die "'$marker' not found";

print "found '$marker' in '$1' \n";
[download]

Output (still no  .marker file):

c:\@Work\Perl\monks\1nickt\two\one\zero>perl find_marker_2.pl
at level 'c:/@Work/Perl/monks/1nickt/two/one/zero'
at level 'c:/@Work/Perl/monks/1nickt/two/one'
at level 'c:/@Work/Perl/monks/1nickt/two'
at level 'c:/@Work/Perl/monks/1nickt'
at level 'c:/@Work/Perl/monks'
at level 'c:/@Work/Perl'
at level 'c:/@Work'
at level 'c:'
'.marker' not found at find_marker_2.pl line 62.
[download]

Both versions of code behave the same if a  .marker is present at some level, although the first version will fail if the  .marker file is present above the level of, say, the  1nickt directory.