It's too late to set the env vars in the begin block, they must be set in the process that runs Perl itself.

This works:

PERL_PERTURB_KEYS=0 PERL_HASH_SEED=1 perl -lE 'say for keys %{ { qw( a
+ 1 b 2 c 3 d 4 ) } }'
c
d
a
b
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If you really insist on doing it from Perl itself, the following seems to work:

#! /usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

if (($ENV{PERL_PERTURB_KEYS} // 1) != 0 || ($ENV{PERL_HASH_SEED} // 0)
+ != 1) {
    $ENV{PERL_PERTURB_KEYS} = 0;
    $ENV{PERL_HASH_SEED} = 1;
    exec $^X, $0, @ARGV
}

say for keys %{ { qw( a 1 b 2 c 3 d 4 ) } }, @ARGV;
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