Since running a command requires a separate process, fork it and set the environment variable via %ENV, then exec your program:

if (fork == 0) {
 local $ENV{myvariable} = "test";
 exec("echo ".($^O eq "MSWin32" ? "%myvariable%" : '$myvariable')) or 
+die "exec: $!\n";
}
__END__
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