This avoids the error for me:

my $cmd = 'find . -type f \( -perm -0020 -o -perm -0004 -o -perm -0002
+ -o -perm -0001 -o -perm -1000 \) -ls';
my $out = "output.txt";
open (MYFILE,"> $out");
print MYFILE `$cmd`;
[download]

See also:

• File::Find