my $x = ( 1 << 54 ) | 1;
say $x;  # 18014398509481985

$x = unpack 'F', pack 'F', $x;
$x = unpack 'J', pack 'J', $x;

say $x;  # 18014398509481984

[download]

To support negative numbers, use I instead of J for negative numbers.

The following is a little bit truer to a cast, but it's far more complicated, far more fragile (can fail for overloaded and magical vars), and technically uses XS: 

use strict;
use warnings;
use feature qw( say );

use B qw( svref_2object );

my $x = ( 1 << 54 ) | 1;
say $x;  # 18014398509481985

# Add the value as an NV to the scalar.
{ no warnings qw( void ); log $x; }

# Make it so the scalar contains just an NV.
$x = svref_2object(\$x)->NV;

# Add the value as an IV or UV to the scalar.
{ no warnings qw( void ); $x | 1; }

# Make it so the scalar contains just an IV or UV.
$x = svref_2object(\$x)->int_value;

say $x;  # 18014398509481984

[download]

In reply to Re^3: Detecting whether UV fits into an NV by ikegami
in thread Detecting whether UV fits into an NV by syphilis

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