thanks the doc for Encode::Mime::Header explained what i was doing wrong

#my $string = "=?UTF-16?B?dwAgAEMAcgBlAGQAaQB0ACAAQwBhAHIAZAAgAEMAbwB1
+AGwAZAAgAEIAZQAgAEgAZQBhAGQAZQBkACAAWQBvAHUAcgAgAFcAYQB5AA==?=";
my $string = "dwAgAEMAcgBlAGQAaQB0ACAAQwBhAHIAZAAgAEMAbwB1AGwAZAAgAEIA
+ZQAgAEgAZQBhAGQAZQBkACAAWQBvAHUAcgAgAFcAYQB5AA==";

print MIME::Base64::decode( $string ), "\n";

w Credit Card Could Be Headed Your Way
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i had to use base64 decoding on the encoded word, and not the whole string