You want unpack "B*" (not unpack "b*"), and it requires packed bytes.

$ perl -Mv5.14 -e'
   my $scabyte = 0x41;
   say unpack "B*", pack "C*", $scabyte;
'
01000001
[download]

When you already have unpacked bytes, you can use sprintf "%b".

$ perl -Mv5.14 -e'
   my $scabyte = 0x41;
   say sprintf "%08b", $scabyte;
'
01000001
[download]

---

#!/usr/bin/perl
use v5.14;
use warnings;

my $packed_bytes = "ABCD";
my @bytes = unpack 'C*', $packed_bytes;

my $bin = '';
for my $byte ( @bytes ) {
   printf "Decimal = [%1\$d]  Hex = [%1\$02X]  Binary = [%1\$08b]\n",
      $byte;

   $bin .= sprintf( "%08b", $byte );
}

say "";
say $bin;
[download]

Decimal = [65]  Hex = [41]  Binary = [01000001]
Decimal = [66]  Hex = [42]  Binary = [01000010]
Decimal = [67]  Hex = [43]  Binary = [01000011]
Decimal = [68]  Hex = [44]  Binary = [01000100]

01000001010000100100001101000100
[download]

---

#!/usr/bin/perl
use v5.14;
use warnings;

my $packed_bytes = "ABCD";

my $bin = unpack "B*", $packed_bytes;

say $bin;
[download]

01000001010000100100001101000100
[download]