If the main script is being run as root, you can easily switch user by assigning to $< and $>

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";
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