The string 0x00000019 is indeed not a number. The integer produced by Perl code 0x00000019 is a number (25), but the string is being passed to pack, not a Perl parser. hex and oct can convert such strings to numbers.

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

use Scalar::Util qw( looks_like_number );

my $i = 0x00000019;
my $s = '0x00000019';

say $i;
say $s;

say '---';

say looks_like_number($i) ? 'yes' : 'no';
say looks_like_number($s) ? 'yes' : 'no';

say '---';

say 0+$i;
say 0+$s;

say '---';

say $i =~ /^0/ ? oct($i) : $i;
say $s =~ /^0/ ? oct($s) : $s;
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25
0x00000019
---
yes
no
---
25
Argument "0x00000019" isn't numeric in addition (+) at x.pl line 21.
0
---
25
25
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So you want

pack('C', hex($gBuf{$bName}[$i]))
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