The above responses correctly suggest the /e modifier... though as i interperet OP's needs, "KEYWORD" needs to be used to get the value (i'll assume via a hash, but could be a function call or something else) ...

use strict;
use warnings;
my %values = ( foo => 3, bar => 4, stuff => 5 );
while( <DATA> ){
  s/<:(KEYWORD):(?:([^:>]+:)?>/ $values{$1} * ($2 || 1) /e;
  print;
}
__DATA__
<:foo:4:>
<:bar:5:>
<:bar:>
<:stuff:9:>

# OUTPUT:
12
20
4
45
[download]

one last note to OP: if your intent was like above, but instead of a hash you have variables named $foo, $bar, and $stuff and those are what you want used, then it is possible to do eval('$'.$1), but not recommended -- it is probably better to rework the code to use a hash or function call to look of the value instead.