You need to evaluate the string "6/2" to get the numerical value 3 before you try to do the addition. You can do it using eval but eval on strings is dangerous as it will *execute* the code in the string. This function should be safe to use as it will only do the eval if the string contains + - . 0 1 2 3 4 5 6 7 8 9 / plus spaces and tabs which are harmless. These chars should be all you need for the task at hand....

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
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