You should really be comparing floating point numbers using a threshold, rather than an absolute comparison. The problem in your code (correct me if I'm wrong) is that the representation changed when converting from a string back to a float. The following demonstrates a better way to do this.

my $x = 0.05;
my $y = sqrt($x+$x+$x);
my $g = "$y";
my $h = sprintf "%.20f", $y;

printf "\$y = %.20f\n\$g = %.20f\n", $y, $g;
printf "\$h = %.20f\n", $h;
print "Is \$y equal to \$g? ", floatEqual($y, $g) ? "Yes\n" : "No\n";
print "Is \$y equal to \$h? ", $y == $h ? "Yes\n" : "No\n";

sub floatEqual {
    return (abs($_[0] - $_[1]) <= 10**15);
}
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I also added the $h variable, which works just fine in a direct comparison. I'm sure someone else who knows more about internal representations and conversions can shed more intelligent light on this.