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+# Write a script that computes the equal point in the Fahrenheit and Celsius scales,
+# knowing that the freezing point of water is 32 °F and 0 °C,
+# and that the boiling point of water is 212 °F and 100 °C.
+# This challenge was proposed by Laurent Rosenfeld.
+
+use strict;
+use warnings;
+use v5.10;
+
+# we start by writing a function (to_f) that converts from
+# celcius to farenheit. As it is a linear function, we
+# have to take into account de relationship on increments
+#
+# From the description, we know that a change from 0 to 100
+# celcius corresponde to a change from 32 to 212 farenheit,
+# so 100 C <--> 180 F
+# so to go from C to F we divide by 100, multiply by 180 and
+# add 32.
+
+sub to_f {
+    return (shift) * 9 / 5 + 32;
+}
+
+# now as a response to the challenge, we just try the bisection
+# method https://en.wikipedia.org/wiki/Bisection_method.
+# (We could just solve the ecuation using standard math
+# but that would't be a computer challenge I guess).
+# we start with -1000 and +1000 as endpoints, and
+# 1/1000000 as tolerance
+# 10000 max iterations (although 50 is more than enough for this
+# linear, simple case)
+
+my ( $min, $max ) = ( -1000, 1000 );
+my $tol  = 1 / 1_000_000;
+my $nmax = 10_000;
+my ( $c, $f, $found );
+
+while ( $nmax-- ) {
+    $c = ( $min + $max ) / 2;
+    $f = to_f($c);
+    $found++ && last if abs( $f - $c ) < $tol;
+    my $fmin = to_f($min);
+
+    # now compares signs of ($f - $c) vs ($fmin - $min) to decide
+    # the new interval
+    if ( $f > $c && $fmin > $min || $f < $c && $fmin < $min ) {
+        $min = $c;
+    }
+    else {
+        $max = $c;
+    }
+}
+
+say $found
+  ? sprintf "Solution found: %.3f Centigrades is %.3f Farenheit", $c, $f
+  : "Error: no solution found";