use strict;
my $value=5;
while (my $line=<DATA>){
$line =~ s/<:KEYWORD:([^:>]*)\:?>/my $dum=$1; my $val; if ($dum) { $val = $value * $dum }else{ $val=$value} $val/e;
print $line, "\n";
}
__DATA__
<:KEYWORD:4:>
<:KEYWORD:5:>
<:KEYWORD:>
<:KEYWORD:9:>