$bitfield = ....
my @fields = ( $modifier =~ m{ ([+-])  (\d+) }gxms ); # extract mods
while ( @fields >= 2 ) {                # for each modification
    my $op    = shift @fields;
    my $value = shift @fields;

    if ( $op eq '+') {
        $bitfield |= $value;            # set a bit w/ OR
    }
    elsif ($op eq '-' ) {
        my $dummy;                      # no-op to force full-width
        $dummy = sprintf "%064b\n", $value;
        my $negated = ~$value;
        $trade_condition &= $negated    # clear a bit w/ negated AND
    }
}