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#!/usr/bin/perl
print "effective: $>, real: $<\n";
...
print "effective: $>, real: $<\n";
$< = 0;   # no effect
print "effective: $>, real: $<\n";

effective: 0, real: 0
Can't write bar: Permission denied
effective: 111, real: 111
effective: 111, real: 111

quux [gm] /tmp/foo # ls -l
total 4
-rw-r--r-- 1 root root      0 2006-06-26 02:31 bar
-rw-r--r-- 1  111 root      0 2006-06-26 02:31 foo
-rw-r--r-- 1 root root    338 2006-06-26 02:29 setuid.pl

   setuid  Sets the real user identifier and the effective user identi
+-
           fier for this process.  Similar to assigning a value to the
           Perl's builtin $< variable, see "$UID" in perlvar, except
           that the latter will change only the real user identifier.