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