Assuming you have gotten $key and $value from the file, you could:

$value =~ s((\$\{(\w+)\}|\$(\w+)))
           ($ENV{$2} or $ENV{$3} or $1)eg;
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This would support an input file with two possible interpolations:

key1=value
key2=value$key1
key3=${key1}value
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It depends on the order of definition. You could iterate the replacement step across all strings until the keys stop changing, but that may raise new problems.

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