Shell-style variable interpolation has been discussed in recent nodes. The one you have above won't work at all like you seem to expect.

PATH = /someplace
PATH = $PATH:/somewhere/else
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In this case, $env_var{PATH} would equal ":/somewhere/else/someplace".

I'm afraid I can't find the original thread, but a fairly robust regular expression looked something like this:

$value =~ s/$({?)(\w+)(?(1)})/$env_var{$2}/eg;
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This would interpolate stuff like $PATH, ${PATH}, etc.