Neat and powerful.

One quibble-- if you had a non-word character in there, like so:

foo=hello
fruit=apple
foo.bar=tunafish
color=red
baz=$foo.bar
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Then it would turn it into

baz=hello.bar
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instead of

baz=tunafish
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Still, it's a powerful technique, and I like it. It gives config-files a Makelike capability. You could say:

SCRIPT_DIR=/usr/local/scripts
MAIN_SCRIPT=$SCRIPT_DIR/main_script.pl
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and then get:

MAIN_SCRIPT=/usr/local/bin/scripts/main_script.pl
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You could probably answer the above objection by putting bracket support into it, like so (untested):

$value =~ s/\$ ( {\.+?} | \w+)/$config{$2}/gx;
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That way, we could handle my previous example like so:

foo=hello
fruit=apple
foo.bar=tunafish
color=red
baz=${foo.bar}
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