use strict;
use warnings;
use IPC::Run 'run';

my $code = q{
	print "Real user id: $<\n";
	warn "Effective user id: $>\n";
};

my ($stdout, $stderr);
{
	local $< = local $> = 500;
	run [ qw/perl -e/, $code ], '>>', \$stdout, '2>>', \$stderr;
}

print "STDOUT was: $stdout";
print "STDERR was: $stderr";