Keys in hashes are unique. The only way for a key to have multiple values is if the "value" is a reference to some container (an anonymous hash, anonymous array, or some object). Let's say your hash holds references to arrays. You might do this:

my %hash = ( unwanted => [1], wanted => [2, 3, 4] );
my $target_key = 'wanted';
if(    exists $hash{ $target_key } 
    && ref $hash{ $target_key } eq 'ARRAY' 
    && @{ $hash{ $target_key } } > 1 
) {
    print "The element associated with $target_key holds a reference t
+o an array of @{$hash{$target_key}} values.\n";
}
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