You missed that FIELD4 has PICTURE 9(8) which doesn't fit in two bytes.

But, you are right, the modules for BCD do exist, as you expected, however, they are not the easiest to find.

• Convert::IBM390 on CPAN seems to work with EBCDIC only
• Corion's PIC module is in Re: EBCDIC and COBOL records

Meanwhile, dbarkho14 probably has his problem solved, but here's my take on it, anyway:
(test data from Re^2: Perl Unpack Cobol Binary File and Fields)

use 5.011;
use warnings;
use Data::Dumper;

my $rec = "\0\0\1\257\a\344\0\3\0\0\0s\0\6\2\267\0\0\0\0\0\0\5E\35\0\0
+\0\0\0\0\0\0\f";
say "Length is ", length $rec;

my @fields = unpack "l>s>s>l>s>s>H18H18", $rec;
say Dumper @fields;

say Dumper map {
    my ($value, $sign) = unpack "A17A";
    $value = ( $sign =~ /[BD]/i ? -1 : +1 ) * $value;
} @fields[-2 .. -1];
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