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use strict;
...
    my $formula = shift;
    return sub { my $x = shift; return eval($formula);}
}

$ perl solver.pl
x = 0 ; val = -8 ; temp = 2; step = 10
...
x = 7.999999999992724 val = -0.000000000007276 temp = 0.00000000000181
+9 avg = 7.999999999997272
x = 7.999999999997272 val = -0.000000000002728 temp = 0.00000000000181
+9 avg = 7.999999999999545
Result is: 8.0000000000