Note that this substitution does not handle RFC822 addresses which contains elements beyond the local_part@domain atoms (however this may be considered beyond the scope of this question).

Update - Okay, updated using a subset of the RFC822 email address regular expression from Mastering Regular Expressions to strip off the trailing domain portion to leave the "username" element ...

$email =~ s/(?:\@[\040\t]*(?:(?:[^(\040)<>@,;:"\\\[\]\000-\037\x80-\xf
+f]|(?:\[(\040)<>@,;:"\\\[\]\000-\037\x80-\xff]))+(?!(?:[^(\040)<>@,;:
+"\\\[\]\000-\037\x80-\xff]|(?:\[(\040)<>@,;:"\\\[\]\000-\037\x80-\xff
+])))|\[(?:[^\\\x80-\xff\n\015\[\]]|\\[^\x80-\xff])*\])[\040\t]*(?:\([
+^\\\x80-\xff\n\015()]*(?:(?:\\[^\x80-\xff]|\([^\\\x80-\xff\n\015()]*(
+?:\\[^\x80-\xff][^\\\x80-\xff\n\015()]*)*\))[^\\\x80-\xff\n\015()]*)*
+\)[\040\t]*)*(?:\.[\040\t]*(?:\([^\\\x80-\xff\n\015()]*(?:(?:\\[^\x80
+-\xff]|\([^\\\x80-\xff\n\015()]*(?:\\[^\x80-\xff][^\\\x80-\xff\n\015(
+)]*)*\))[^\\\x80-\xff\n\015()]*)*\)[\040\t]*)*(?:(?:[^(\040)<>@,;:"\\
+\[\]\000-\037\x80-\xff]|(?:\[(\040)<>@,;:"\\\[\]\000-\037\x80-\xff]))
++(?!(?:[^(\040)<>@,;:"\\\[\]\000-\037\x80-\xff]|(?:\[(\040)<>@,;:"\\\
+[\]\000-\037\x80-\xff])))|\[(?:[^\\\x80-\xff\n\015\[\]]|\\[^\x80-\xff
+])*\])[\040\t]*(?:\([^\\\x80-\xff\n\015()]*(?:(?:\\[^\x80-\xff]|\([^\
+\\x80-\xff\n\015()]*(?:\\[^\x80-\xff][^\\\x80-\xff\n\015()]*)*\))[^\\
+\x80-\xff\n\015()]*)*\)[\040\t]*)*)*)$//;
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perl -le 'print+unpack"N",pack"B32","00000000000000000000001001111011"'