Oops, I forgot that this requires an extra layer of indirection. I suspect that the number of environments where "I" is the wrong choice is very small. Strangely, I used "L" in Acme::ESP which seems a more fragile choice.

my $fmt= "I";
my $iBytes= length( pack $fmt, $fmt );
my $pBytes= length( pack "P", $fmt );
if(  $iBytes != $pBytes  ) {
    die "Your pointers are $pBytes bytes, ints are $iBytes";
}
my $pointer= pack $fmt, 0x78000000;
my $bytes= 4*1024;                  # Set to number of bytes to copy

my $copy= unpack "P$bytes", $pointer;
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