The documentation for pack says there's no builtin way to do it:

  Real numbers (floats and doubles) are in
  the native machine format only; due to the
  multiplicity of floating formats around,
  and the lack of a standard "network" rep­
  resentation, no facility for interchange
  has been made.  This means that packed
  floating point data written on one machine
  may not be readable on another - even if
  both use IEEE floating point arithmetic
  (as the endian-ness of the memory repre­
  sentation is not part of the IEEE spec).

  See also the perlport manpage.

I don't really know much about the machine format of floating point numbers, but if the only problem is endianness, unpacking as characters, reversing them, putting them back together, and then interpreting them as native format might work. Something like this:

#!/usr/bin/perl

my $f = "3.6";
my $floatpack = pack("d",$f);

my $floatpack2 = pack("c*",reverse unpack("c*",$floatpack));
$f2 = unpack("d",$floatpack2);
print "f2=$f2\n";

my $floatpack3 = pack("c*",reverse unpack("c*",$floatpack2));
$f3 = unpack("d",$floatpack3);
print "f3=$f3\n";
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