'\w' doesn't match the '.' or '-' characters, so the regex fails. It only matches A-Z, a-z, 0-9 and the underscore. If you want to go this route, change at least the 3-8th '\w's to something like [A-Z0-9\.\-]. For even more security and readability, you can construct your regex in a couple extra stages:

  # prepare regex sub-expressions
  my $sizerx = '(-|\d+(\.\d+)?[GM]?)'; # Matches '-', or a float + 'M'
+ or 'G' for size
  my $loadrx = '(-|\d+\.\d\d)'; # Matches '-' or a float
  my $procrx = '(-|\d+)';      # matches '-' or an integer

  # Test the line to see if it matches
  if( $_ =~ /^(\w+)\s+    # host
              (\w+)\s+    # os
              $procrx\s+  # nproc
              $loadrx\s+  # load
              $sizerx\s+  # memtot
              $sizerx\s+  # memuse
              $sizerx     # swapto
              \s*$
            /x 
  ) {
    print "using regex, host is [$1]\n"; 
  }
[download]

For speed, you should define the sub-expressions outside the loop. If you plan to work with the various parts, you could immediately assign the matches to named variables and check for them, like so:

  my($host,$os,$nproc,$load,$memtot,$memuse,$swapto) = (
    $_ =~ /^(\w+)\s+    # host
            (\w+)\s+    # os
            $procrx\s+  # nproc
            $loadrx\s+  # load
            $sizerx\s+  # memtot
            $sizerx\s+  # memuse
            $sizerx     # swapto
            \s*$
          /x
  );
  if(defined $host) {
    print "using regex, host is [$host]\n";
  } else {
    next;
  }
[download]

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