If you are wondering why your code returns "Supported", it is because both arrays contain the 2 strings: "Accounting" and "Dnd". According to the documentation, your grep returns the value of 2, which is true in the if statement:

In scalar context, returns the number of times the expression was true.

It becomes more obvious if you add these lines of debug code:

my $num = grep ($seen{$_}, @SupportedPTs);
print "num=$num\n";
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On another note, you could save yourself some typing by using the qw operator:

my @ProblemTypes = qw(Accounting Dnd test);
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