See substr for getting at the bytes, and then Encode for documentation about the decode function. For decoding COMP fields, I use the following:

sub decode_COMP3 {
    my $digits = unpack "H*", $_[0];
    #warn "COMP-3: $digits\n" if $digits =~ m!^0?404040!;
    if( $digits =~ m!^(?:40)+(?:00|f0)*$!i ) {
        warn "Invalid (space) content for field detected: $digits.";
        $digits=~ s![4f]0!00!g;
    } elsif( $digits =~ /[abcdef]\d/ ) {
        my $old= $digits;
        $digits=~ s![a-f]!0!g;
        warn "Invalid number/misaligned content for field detected [$o
+ld] -> [$digits].";
    };
    my $sign = chop $digits;
       if ($sign eq 'D' or $sign eq 'd') { $sign = '-' }
    elsif ($sign eq 'C' or $sign eq 'c') { $sign = '+' }
    elsif ($sign eq 'F' or $sign eq 'f') { $sign = ' ' }
    elsif ($sign =~ /^\d+$/) { $digits .= $sign; $sign = '' }
    else { $digits .= $sign; $sign = '?' };
    "$sign$digits"
};
[download]

If you have numbers with an implicit decimal point, you need to add that in afterwards.