perl -l -0e 'BEGIN{
@f=(file_3,file_4,file_1);   # define array containing list of files
$N=$#f                       # assign number of files to $N
}
for $i (0..$N){              # iterate over indexes of @f array
for $j (0..$i-1,$i+1..$N){   # once again iterate over indexes of @f array, but excluding the index that $i is holding
open I,"<$f[$j]";            # open file stored under index $j for reading. 
$a.=<I>                      # append that file's contents to $a variable. It is achieved by using -0 option (perl -l -0e), which results in whole file being loaded during single reading.
}
open O,">files${i}.tmp";     # open file "files[number].tmp" for writing
print O $a;                  # write contents of $a variable to that file. So now files[number].tmp contains concatenated contents of all files, excluding the one that is stored under "number" index.
$a=""};                      # clear $a contents
for $i (0..$N){              # iterate over indexes of @f array
print "$f[$i] unique\n";     # print message
system "bash -c \"comm -23 <(sort $f[$i]) <(sort files$i.tmp); # execute external "comm" command using contents of file stored under $i index, and concatenated contents of all the other files (files$i.tmp).
rm -f files$i.tmp\"";        # it is part of system statement too
print "\n##############\n"   # and this is obvious I think
}'