Well, yes. At the time the variable substitution occurs, the variable has not been set yet. eg.:

$ quux=foo echo $quux

$
[download]

"Fix" it by separating the one command into 2:

$ quux=foo; echo $quux
foo
$
[download]

It isn't just bash, BTW. Any Bourne-compatible shell should behave like this.

You would still have to export it to see it in the perl process:

$ export M=MyMod
$ perl -M$M -E 'say $ENV{M}->VERSION'
$ unset M
[download]

Or just do it in a subshell to keep your environment clean:

$ (export M=MyMod; perl -M$M -E 'say $ENV{M}->VERSION')
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yes my point was to avoid the export, because it IS visible inside the Perl process

~$ M=whatever perl -E'say $ENV{M}'
whatever
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unfortunately a dynamic use inside the process is not less complicated than the solutions already shown (eval or Ikegami's if 1 trick)

Cheers Rolf
_{(addicted to the Perl Programming Language :)
see Wikisyntax for the Monastery}