my $equ= $q->param("equation");
my( $var, $expr )= $equ =~ /^\s*(\w+)\s*=(.*)/
  or  fail("Invalid equation");
my @words= qw( x y z abs atan2 cos exp log sin sqrt );
my %words;
@words{@words}= (" ") x @words;
( my $check= $expr ) =~ s/(\w+)/ $words{$1} ? "" : "x" /ge;
if(  $check =~ m#([^-\s\d+*/()]+)#  ) {
    fail( "Disallowed function/variable ($1)" );
}
@words{qw(x y z)}= qw( $x $y $z );
$expr =~ s/(\w+)/ $words{$1} /ge;
my $sub= eval 'sub { my($x,$y,$z)= @_; return ' . $expr . '}';
fail( "Invalid expression ($expr): $@" )  if  $@;