Just for fun, this version detects which of the two files is actually missing:

use strict;
use warnings;

my %file;
while(<DATA>) {
  chomp;
  if( !/^lab(\d+)_set(\d+)\.txt/ ) {
    warn "$_ doesn't belong here!";
    next;
  }
  $file{$1} += $2;
}

for( keys %file ) {
  print
    "lab$_ ",
    $file{$_} == 3
      ? 'matched'
      : "is missing lab${_}_set" . ( 3 - $file{$_} ) . ".txt",
    "\n";
}


__DATA__
lab1_set1.txt
lab1_set2.txt
lab2_set1.txt
lab2_set2.txt
lab3_set2.txt
lab100_set1.txt
lab100_set2.txt
[download]

...produces...

lab1 matched
lab3 is missing lab3_set1.txt
lab100 matched
lab2 matched
[download]