This code will run not just a "little bit" faster than a Win command line...the performance difference is HUGE, even with just 8K buffer.

That's some strange code and a big claim. I thought I test the claim and my first attempt to call bcopy copy got:

Too many arguments for main::bcopy at C:\test\junk8.pl line 34, near "
+@in )"
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Once I removed the useless prototype:

#! perl -sw
use 5.010;
use strict;

sub bcopy
{
   (my $out, my @in_list)=@_;
   open (OUTBIN, ">", "$out") || showfailed
                                ("unable to open $out");
   binmode(OUTBIN)        || showfailed
                              ("unable to set binmode $out");

   foreach my $infile (@in_list)
   {
       open(INBIN, "<", "$infile")|| showfailed
                                      ("unable to open $infile");
       binmode(INBIN)           || showfailed
                                    ("unable to set binmode $infile");
       while (read(INBIN, my $buff, 8 * 2**10))
       {
          print OUTBIN $buff;
       }
       close(INBIN) || showfailed("unable to close $infile");
       print "$infile appended to $out\n";
   }
   close(OUTBIN)  || showfailed("unable to close $out");
}

my @in = glob shift;
my $out = shift;

say time;

bcopy( $out, @in );

say time;
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and ran it on 10x 128meg files:

[10:11:26.67} C:\test>junk8 *.jnk bigjnk.out
1247908624
bugjunk1.jnk appended to bigjnk.out
bugjunk10.jnk appended to bigjnk.out
bugjunk2.jnk appended to bigjnk.out
bugjunk3.jnk appended to bigjnk.out
bugjunk4.jnk appended to bigjnk.out
bugjunk5.jnk appended to bigjnk.out
bugjunk6.jnk appended to bigjnk.out
bugjunk7.jnk appended to bigjnk.out
bugjunk8.jnk appended to bigjnk.out
bugjunk9.jnk appended to bigjnk.out
1247908656
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32 seconds. Then again with xcopy:

[10:22:58.79} C:\test>xcopy /Y *.jnk bigjnk.out
Does bigjnk.out specify a file name
or directory name on the target
(F = file, D = directory)? f
C:bugjunk1.jnk
C:bugjunk10.jnk
C:bugjunk2.jnk
C:bugjunk3.jnk
C:bugjunk4.jnk
C:bugjunk5.jnk
C:bugjunk6.jnk
C:bugjunk7.jnk
C:bugjunk8.jnk
C:bugjunk9.jnk
10 File(s) copied

[10:23:06.51} C:\test>
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Even with time it took me to respond to the dumb prompt, took just 8 seconds.