use Config;

die "Int/double size mismatch" unless $Config{ ivsize } == $Config{ nvsize };

##</code><code>##

$packedDoubleAsInt = unpack 'Q', pack 'd', 3.14159265358979;;

print $packedDoubleAsInt;;
4614256656552045841

$unpacked = unpack 'd', pack 'Q', $packedDoubleAsInt;;

print $unpacked;;
3.14159265358979

$unpacked = unpack 'd', pack 'Q', 4614256656552045841;;

print $unpacked;;
3.14159265358979