here is a very simple example of how this might be done.

$text = 'speed of light in km/fortnight';

if ($text =~ /(.*)\s+in\s+(.*)/) {
   print "1: ($1) ($2)\n";
}


$text = '2+8/2*3';

while (length $text) {
    if ($text =~ m|(\d+)\s*([/*])\s*(\d+)|) {
print "($1) $2 ($3)\n";
        $x = eval " $1 $2 $3 ";
        $text =~ s|\d+\s*[/*]\s*\d+|$x|;
    } else {
        last;
    }
}
while (length $text) {
    if ($text =~ m|(\d+)\s*([+-])\s*(\d+)|) {
print "($1) $2 ($3)\n";
        $x = eval " $1 $2 $3 ";
        $text =~ s|\d+\s*[+-]\s*\d+|$x|;
    } else {
        last;
    }
}
print "answer $text\n";
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This will not work for your example because it does not recognise floating point numbers.