/^(?<!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]

In reply to Re: Driving the Engine right off the road by tybalt89
in thread Driving the Engine right off the road by Intrepid

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