This works for me:

#!/usr/bin/perl

use MIME::Parser;

my $parser = MIME::Parser->new;

# Open temp file for both read and write
open (TMP, "+> /tmp/copy.txt")
    or die "Couldn't open copy for write: $!\n";

# Copy the whole thing
while (read(STDIN,$buf,8192))
{
  print TMP $buf;
}

# Rewind the temp file
seek(TMP,0,0)
    or die "Seek error: $!\n";

# Now create our MIME entity.
my $entity = $parser->parse(\*TMP)
    or die "Couldn't parse: \n";
close(TMP)
    or die "Error closing temp file: $!\n";

# Done!
$entity->dump_skeleton;
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