sub frac2dec {
    my $str = shift;
    $str = eval $str if $str =~ m!^[\-\+\d\./ \t]+$!;
  $str;
}

$a = "  6/2";
$b = " 4 / 8 ";
print frac2dec($a) + frac2dec($b);

__DATA__
3.5