I tested four cases:

perl -le 'print $^X'
/usr/bin/perl -le 'print $^X'
/usr/bin/../bin/perl -le 'print $^X'
./prrl -le 'print $^X'  ## symbolic link
[download]

The path returned by the program was absolute in all cases on:

• Debian GNU/Linux 3.1r0, perl 5.8.4
• Red Hat Enterprise Linux ES release 4, perl 5.8.5
• SuSE Linux Enterprise Server 10.0, perl 5.8.8
• Windows XP Professional SP2, perl 5.8.8 ActiveState build 817 (symlink not tested)

In these systems, the program simply returned the same path used for invoking perl:

• FreeBSD 5.4, perl 5.8.8
• FreeBSD 6.0, perl 5.8.8
• HP-UX 11i 11.11, perl 5.005_02
• HP-UX 11i v2, perl 5.8.3 ActiveState build 809

Not yet tested for various reasons:

• NetBSD 3.0
• HP Tru64 Unix 5.1b

Update: Added fourth test. Added SuSE Linux.