How will your Perl program know that it has already re-executed itself?

My approach would be to set a second environment variable to tell your program that it is already re-executing:

warn "$0 launched";
if (! $ENV{RE_EXECUTING}) {
    $ENV{RE_EXECUTING} = 1;
    warn "Set up environment. Re-executing now.";
    exec ...
};
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But maybe you want to just check whether the environment needs changing at all, and only if you need changes re-execute?