Personally I think $float = unpack 'f', pack 'L', unpack 'N', $data; is a better solution. It is conforming to the standard, ie, it acknowledges that the input data is in network long format. It is more portable, although you assume that float is 32 bits long. Consider the packing and unpacking process -

 packing float as network long:
 +-----+    +-----------+    +------------+
 |float|--->|native long|--->|network long|
 +-----+    +-----------+    +------------+

 and unpacking network long back to float
 +------------+    +-----------+    +-----+
 |network long|--->|native long|--->|float|
 +------------+    +-----------+    +-----+
[download]

where perl takes care of translation between native long (big or little endian) and network long (always big endian), without you having to worry explicitly what is the underlying architecture.

The $float = unpack "f", pack "N", unpack "V", $data; and the $num = unpack "f", reverse $packed_num; solutions both assume that you are on a little endian machine, making you dependent to the underlying architecture.

Update: I have written the following test program to test this solution-

use strict;
use IO::File;

savefloat();  # <--- this is run from Sun E10000
readfloat();  # <--- this is run from Windows XP

sub savefloat {
  my $original_float = 1234.5678;
  my $network_long = pack 'N', unpack 'L', pack 'f', $original_float;
  my $f = new IO::File "NetworkLong.txt", "w";
  print $f $network_long, "\n";
}

sub readfloat {
  my $f = new IO::File "NetworkLong.txt", "r";
  chomp(my $network_long = <$f>);
  my $float = unpack 'f', pack 'L', unpack 'N', $network_long;
  printf "%0.4f\n", $float;
}
[download]

Two separate tests were conducted -

Test 1

On SUN: float->network_long->disk
On WindowsXP: disk->network_long->float
[download]

Test 2

On WindowsXP: float->network_long->disk
On SUN: disk->network_long->float
[download]

I got the same result in both test 1 and test 2. This proves that the particular packing method worked cross-platform on both Sun and WindowsXP, which are big endian and little endian platforms respectively.