If you run the program, it tells you itself what's wrong:

Applying pattern match (m//) to %hash will act on scalar(%hash) at foo
+.pl line 10.
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What it tells you is that pattern matches generally act only on strings, not on lists or hashes.

You could write your code as

my %modules = %{ $Module::CoreList::version{ 5.008 } };

if( grep /cgi/i, keys %modules ) {
    print "Found one!\n";
}
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instead.