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my $sub= eval "sub { my($x)=@_; return ($x/2)**4 }";
die $@   if  $@;
for my $x (  0..100  ) {
...
    my $y= $sub->( $x );
    plot( $x, $y );
}

if(  $equ =~ m#[^-\s\d+*/()]#  ) {
    # refuse to use it

my $equ= $q->param("equation");
my( $var, $expr )= $equ =~ /^\s*(\w+)\s*=(.*)/
  or  fail("Invalid equation");
...
$expr =~ s/(\w+)/ $words{$1} /ge;
my $sub= eval 'sub { my($x,$y,$z)= @_; return ' . $expr . '}';
fail( "Invalid expression ($expr): $@" )  if  $@;