/^(?<!XDG_) [_A-Z0-9]+ _PATH$/x does not make sense to me, you are trying to look before the beginning of the string.

How about:

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11166082
use warnings;

my @patterns = qw/PKG_CONFIG_PATH XDG_DATA_DIRS XDG_SEAT_PATH XDG_SESS
+ION_PATH/;

my $re = qr/^(?!XDG_ [_A-Z0-0]+ _PATH$)/x;

for ( @patterns )
  {
  print /$re/ ? "   match" : "no match", " $_\n";
  }
[download]

Outputs:

   match PKG_CONFIG_PATH
   match XDG_DATA_DIRS
no match XDG_SEAT_PATH
no match XDG_SESSION_PATH
[download]