comment on

Standard input is already opened for your as STDIN, so

# slurp the email from STDIN
my( $raw );
{ local $/ = undef; local *FILE; open FILE, "-"; $raw = <FILE>; close 
+FILE }
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is an overly complex way of doing

# slurp the email from STDIN
my $raw; { local $/; $raw = <STDIN>; }
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Why are you forcing a list assignment?

my( $raw_iohandle ) = new IO::Scalar( \$raw );
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should be

my $raw_iohandle = new IO::Scalar( \$raw );
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But then again, IO::Scalar is not nearly as robust as the native support for scalar handles added to 5.8

open(my $raw_iohandle, '<', \$raw );
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Finally, if the module expects a system file handle (e.g. if it attempts a child process to read from it), it'll be disappointed by your attempts since none of the Perl file handles you are producing have a system file handle associated with it.

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