$ echo "19284732 Test message 123456789012345" | perl -pe "s/(\d{8,})/scalar localtime $1/e;"
Fri Aug 15 12:52:29 2014 Test message 123456789012345

$ echo "19284732 Test message 123456789012345" | perl -pe 's/(\d{8,})/scalar localtime $1/e;'
Tue Aug 11 23:52:12 1970 Test message 123456789012345