# First stage:

sub process_perl {
    my( $code ) = @_;
    my $res = eval $code;
    my $err = $@;

    warn "Whoops. Caught error [$err] for code [$code]"
        if $err;

    return $res;
};

$template =~ s!<%([^%]+)%>!process_perl("$1")!gse;