Lookahead/behind are zero-width assertions; you are asserting that after matching ^ (beginning of string), what comes just before that can't be XDG_, and then continuing to match the rest of your regex. There are two problems here: first, the beginning of the string will never follow XDG_, so your assertion will always pass; and second, you will only match if the string ends with _PATH or _PATH followed by a newline, which doesn't meet your requirements.

If perl supported variable length lookbehind, you could use a lookbehind at the end: 
/^[_A-Z0-9]+$(?<!^XDG_.*_PATH)/

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but it doesn't. So use a lookahead instead, to test that the entire string is the expected characters but doesn't match your exclusion at the beginning: 
/^(?!XDG_.*_PATH$)[_A-Z0-9]+$/

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or (and this is almost always the more readable approach), do it in code: 
/^[_A-Z0-9]+$/ && ! /^XDG_.*_PATH$/

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

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