$ cat sample.txt | perl -wnl -e '/\.X\-Intermail\-Unknown\-MIME\-Type\
+=unparsedmessag(e.*)/s and print $1;'
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You need to slurp the entire file and you don't need to use cat:

$ perl -0777nle '/\.X-Intermail-Unknown-MIME-Type=unparsedmessage(.*)/
+s and print $1;' sample.txt
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