Changing the print loop to:

for my $key (keys %mappings) {
    for (keys %{$mappings{$key}}) {
        next if $mappings{$key}{$_} < 2;
        print "$key - $mappings{$key}{$_} -> $_\n" ;
    }
}
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Prints:

lung cancer - 2 -> some cancer
cancer - 4 -> foot cancer
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for the code I posted. Note that in the original code for keys %{$mappings{$key}} is a statement modifier and the if statement that you inserted is outside the inner loop so $_ is bogus. The code needed to be changed around into a full for loop so the condition can be checked inside the inner loop.