C:\>perl -wle "print unpack 'H*', pack 'D', 2.4;"
9a999999999999990040000000000000

##</code><code>##

C:\>perl -wle "print unpack 'H*', pack 'D', 2.4;"
00989999999999990040000000000000

##</code><code>##

use warnings;

$template = 'D<';
$nv = 2.4;
$p = pack $template, $nv;

$s = "'$p'";

# Now pass $p to the 'double' build of perl.
$double_perl = "C:/perl-5.34.0/bin/MSWin32-x64-multi-thread/perl.exe";
system $double_perl, '-wle', "print unpack('H*', $s);";

##</code><code>##

use warnings;

$p = pack("D", 2.4);

open WR, '>', 'packstr.txt' or die "Opening: $!";
binmode(WR);
print WR $p;
close WR or die "Closing: $!";

##</code><code>##

use warnings;

open RD, '<', 'packstr.txt' or die "Opening: $!";
binmode(RD);
$p = <RD>;
close RD or die "Closing: $!";

print unpack("H*", $p);