If you know that the only difference is byte order (and not bit order), you can just juggle the bytes manually. As an example, a file generated on my x86 (little-endian) box:

$ perl -we 'my $pi = 4*atan2(1,1);
            print pack "f", $pi' > le-pi.bin
$ od -h le-pi.bin 
0000000 0fdb 4049
0000004
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To read that in on my ppc (big-endian) system, I can do:

$ perl -we 'my $buf;
            read STDIN, $buf, 4;
            my $pi = unpack "f", reverse $buf;
            print "pi=$pi\n";' < le-pi.bin
pi=3.14159274101257
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Doing this to just one part of a larger buffer just takes a bit more imagination (although the fact that substr can be used as an lvalue might make it amusing. Untested, but does this work?)

  # reverse endianness of first 4-byte float:
  substr( $buf, 0, 4 ) = reverse substr( $buf, 0, 4 );
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