I'm probably misunderstanding you, but even if perl is invoked without explicit path info and is found through the path environment variable, $^X is still correctly set:

C:\test>perl -le" print $^X"
C:\Perl\bin\perl.exe

C:\test>..\perl\bin\perl -le" print $^X"
C:\Perl\bin\perl.exe
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The only way I know of getting a different value for the path in $^X is if it is invoked via a subst (or NET USE) drive:

C:\test>subst p: c:\perl\bin

C:\test>p:perl -le" print $^X"
P:\perl.exe
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Even then, so long as the subst remains in force, $^X remains valid for all normal uses.