If your Perl can handle quads in pack, you should be able to just use them, no bigint needed:

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

my $hash = '18165163011005162717';
my $signed = unpack 'q', pack 'Q', $hash;  # Using 'q' woudl work, too
+.
say $signed eq "-281581062704388899";
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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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One follow up question - how does one convert a numeric value (eg -281581062704388899) to base64 in perl similar to java's call:

Base64.getEncoder().withoutPadding().encodeToString(Longs.toByteArray(val));

to get the java base64 encoded string: '/Befg+4HJN0'

See MIME::Base64 and perhaps pack.

Update: Although I do not receive the same result - are you sure about the input and output?

use strict;
use warnings;
use MIME::Base64 'encode_base64';

use Test::More tests => 1;

is (encode_base64((pack 'q', -281581062704388899), ''), '/Befg+4HJN0')
+;
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gives '3SQH7oOfF/w=' instead.

Update: Sorry, posted in the wrong place, this was supposed to be an answer to the OP (see below)

thanks!

There is nothing wrong with cross posting questions, but it is advised to inform the reader in order to avoid duplication of work in various parts of the Internet.